Objective lens, optical pickup, and optical information processing apparatus using the same

ABSTRACT

An optical pickup includes a first optical source producing a first optical beam having a wavelength λ 1 , a second optical source producing a second optical beam having a wavelength λ 2  (λ 1&lt;λ2 ), and a single objective lens focusing any of the first and second optical beams to an optical recording medium, the first optical beam is incident to the objective lens as a parallel beam and the second optical beam is incident to the objective lens as a divergent optical, wherein an aperture is provided at a front side of the objective lens with offset from a principal point at the front side by a distance t given by an equation
 
 t=L−NA 1 ·f /tan( a  sin( NA 2 obj )),
 
wherein f represents a focal distance of the objective lens, NA 1  represents the numerical aperture at the side of the image surface when the first optical source is turned on, NA 2   obj  represents the numerical aperture at the front side when the second optical source is turned on, and L represents the object distance at the time when the second optical source is turned on.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/875,304, filed on Jun. 25, 2004 now U.S. Pat. No. 7,313,074, thesubject matter of which is incorporated in its entirety by referenceherein.

BACKGROUND OF THE INVENTION

The present invention generally relates to optical informationprofessing and more particularly to an objective lens and an opticalpickup for conducting at least one of information recording, informationplayback and erasing of information to optical media designed for bluewavelength band, red wavelength band or infrared wavelength band, aswell as the information processing apparatus that uses the same.

For the means for saving video information, audio information or data ina computer, the use of optical recording media such as a CD having arecording capacity of 0.65 GB, or a DVD having a recording capacity of4.7 GB, is spreading. In these days, there is an acute demand forfurther improvement of recording density for more recording capacity.

In order to increase the recording density in such an optical recodingmedium, it is necessary to increase the numerical aperture (NA) value ofthe objective lens or reduce the wavelength of the optical source in theoptical pickup used for writing or reading information to and from theoptical recording medium, such that the size of the beam spot formed onthe optical recording medium by the objective lens is reduced.

In the case of the optical recording medium called CD, a numericalaperture of 0.40-0.50 is used for the objective lens and a wavelength ofabout 785 nm is used for the optical source. In the case of therecording medium called DVD, in which the recording density is increasedover the CD, a numerical aperture of 0.60-0.65 is used together with thewavelength of the optical source of about 660 nm.

As noted before, there is a keen demand for improvement of recordingdensity and increase of recording capacity in the optical recordingmedium in these days, and thus, there is a desire to increase thenumerical aperture of the objective lens beyond the conventional valueof 0.65 and/or to decrease the wavelength of the optical source to lessthan 660 nm.

Thus, there appears a situation in which a new specification of highernumerical aperture value or shorter wavelength appears every year forthe optical pickup, while the users of the optical informationprocessing apparatus maintain CDs and DVDs of conventionalspecification. Because of this, there is arising a demand that suchconventional optical recording media can be processed also in theoptical information processing apparatus designed for such novel opticalrecording media of novel specification.

While such compatibility between different specifications can beachieved by providing a conventional optical pickup in addition to theoptical pickup designed according to the new specification, such aconstruction is contradictory to the requirement of downsizing and costreduction.

It is preferable to provide a construction shown in FIG. 1A in which ablue optical source 100 and a DVD optical source 200 use a singleobjective lens 104 that focuses the optical beams of the respectiveoptical sources to an optical recording medium 103, such that theoptical pickup, designed for a large-capacity (blue) optical recordingmedium by using the optical source 100 of the blue wavelength band, isstill compatible with the conventional CDs or DVDs. Alternatively, it ispreferable to provide a construction shown in FIG. 1B in which a blueoptical source 100, a DVD optical source 200 and a CD optical source 300cooperate with the single objective lens 2 that focuses the exit beamsof the respective optical sources to the optical recording medium 1.

On the other hand, in order to focus the optical beams to the opticalmedia designed for various different specifications such as blue colorsystems, DVDs or CDs, there arise various problems in relation to thedifference in the wavelength/substrate thickness of the opticalrecording medium 103 as will be explained hereinafter.

Japanese Laid-Open Patent Publication 2002-107617 describes the problemof aberration (see FIG. 3) arising in the case an objective lens 110 ofFIG. 2 designed of the optical source wavelength of 405 nm is used inthe wavelength range of 400-800 nm. In FIG. 2, the reference numeral 111shows an optical recording medium.

As will be explained below, it is generally required that the wavefrontaberration should be 0.02 λrms or less. On the other hand, FIG. 3 showsthat there appears an aberration of 0.10 λms or more at the wavelengthof 660 nm or 785 nm used with a DVD system or CD system. In view of thissituation, the foregoing Japanese Laid-Open Patent Publication2002-107617 proposes the use of a compound lens for the objective lens.However, the use of such a compound lens raises the problems in that itrequires a bonding process at the time of manufacturing of the objectivelens and there arises the problem of increase of weight associated withthe construction that uses two lenses.

As an alternative, there is a proposal in ISOM2001 Abstract,“BLUE/DVD/CD COMPATIBLE OPTICAL HEAD WITH THREE WAVELENGTHS AND AWAVELENGTH SELECTIVE FILTER”, or in Japanese Laid-Open PatentApplication 2003-67972 to construct the respective optical paths of DVDor CD by a finite optical system in view of the fact that there occurs aspherical aberration in the event an optical beam is focused to theoptical recording medium of DVD or CD by the objective lens designed fora blue optical beam. In this proposal, there is further provided a phasecompensation element such that the phase distribution is changed for theoptical beam of DVD or CD while not causing any change of phasedistribution for the blue optical beam of the blue wavelength band. Byusing a finite optical system, in which the incident optical beam to theobjective lens is made divergent, an effect of reducing the sphericalaberration is achieved. However, there still remains some sphericalaberration, and compensation of this residual spherical aberration isachieved by inducing a spherical aberration of opposite phase by thephase compensating element.

For example, in the case of using a single objective lens, which isdesigned for minimizing the wavefront aberration when it is used with aninfinite optical system (a parallel beam is incident to the objectivelens) and with a blue optical recording medium (λ1=407 nm, NA(λ1)=0.67,substrate thickness t1=0.6 mm), for focusing a beam spot on a DVDrecording medium (λ2=660 nm, NA(λ2)=0.65, substrate thickness t2=0.6mm), there arises a spherical aberration shown in FIG. 12B due to thedifference in the wavelength.

Such a problem was also noticed in the DVD/CD compatible optical pickup.Thus, in the case of using a single objective lens, which is designedfor minimizing the wavefront aberration when it is used with an infiniteoptical system (a parallel beam is incident to the objective lens) andwith a DVD recording medium (λ2=660 nm, NA(λ2)=0.65, substrate thicknesst2=0.6 mm), for focusing a beam spot on a CD recording medium (λ3=780nm, NA(λ3)=0.50, substrate thickness t3=1.3 mm), there arises a similarspherical aberration shown due to the difference in the wavelength.

In order to attend to this problem, Japanese Patent 2,725,653 orJapanese Laid-Open Patent Application 10-334504 teaches the constructionin which there are provided two laser diodes of different wavelengthsand a phase compensating element having wavelength selectivity, suchthat recording or playback is conducted for a DVD optical recordingmedium having the thickness of 0.6 mm by using a beam of 660 nmwavelength emitted from one of the laser diodes and carries outrecording or playback for a CD optical recording medium having athickness of 1.2 mm by using the beam of 780 nm wavelength emitted fromthe other laser diode. Thereby, the phase compensating element havingthe wavelength selectivity causes no change of phase distribution forthe optical beam of the 660 nm wavelength, while the phase compensatingelement causes a phase distribution change for the optical beam of the780 nm wavelength in such a manner that the spherical aberration causedby the difference in the thickness of the substrate is successfullycompensated for.

As an alternative, there is generally known the means of compensatingfor the spherical aberration caused by difference in the thickness orwavelength between the DVD optical recording medium and the CD opticalrecording medium, by using an infinite optical system when the opticalbeam of the 660 nm wavelength for DVD is incident to the objective lensand by using a finite optical system (a state in which a divergent beamenters into the objective lens in the form of a divergent beam) for thecase the optical beam for CD is incident to the objective lens.

Particularly, the ISOM2001 Abstracts op. cit., proposes the method ofrecording or playing back information to or from any of the threeoptical recording media of blue optical recording medium, DVD opticalrecording medium and CD optical recording medium while using a singleobjective lens. According to the teaching of this prior art, there isprovided a system including three different laser diodes of thewavelengths of 405 nm, 650 nm and 780 nm and a wavelength-selectivephase compensating element having wavelength selectivity, such thatoptical irradiation is made to a blue optical recording medium having athickness of 0.1 mm with the wavelength of 405 nm by using an infiniteoptical system, and an optical irradiation is made to a DVD opticalrecording medium having a thickness of 0.6 mm with the wavelength of 660nm by using a finite optical system, and optical irradiation is made toa CD optical recording medium having a thickness of 1.2 mm with thewavelength of 780 nm by using a finite optical system. Thereby, thewavelength-selective phase plate does not change the phase distributionfor the optical beam of the wavelength of 405 nm while it changes thephase distribution for the optical beams of 660 nm and 780 nm. In thisconstruction, the wavelength-selective phase compensating means and thefinite optical system used for the case of the DVD/CD recording mediaconstitute the wavefront compensating means compensating for thespherical aberration caused by the difference in the thickness of thesubstrate.

In addition to the foregoing problem of aberration, there arises anotherproblem in that the numerical aperture value changes between the opticalrecording media of different specifications.

Generally, the numerical aperture values NA of 0.60 and 0.65 are usedrespectively for the playback system and recording system of a DVDoptical recording medium, while the numerical aperture values NA of 0.45and 0.50 are used respectively for the play back system and recordingsystem of a CD optical recording medium.

Most of the optical information processing apparatuses currently soldare capable of recording for both of DVD and CD, and it is preferable touse the numerical aperture value NA of 0.65 for DVD and the numericalaperture value NA of 0.50 for CD.

On the other hand, with regard to the blue optical recording technology,there is proposed the use of the numerical aperture value NA of 0.65 inrelation to the HD-DVD technology.

This means that there is a need of switching the numerical aperturevalue in response to the optical recording medium used for recording andplayback.

Thus, in order to achieve two-generation compatibility between DVD andCD, there is proposed a technology that uses an aperture switchingelement as disclosed in the Japanese Patent 3,240,846, Japanese Patent2,713,257, Japanese Patent 2,725,653, and Japanese Patent 3,036,314.Further, in order to achieve two-generation compatibility between alarge-capacity optical recording medium and DVD, there is proposed atechnology that uses an aperture switching element as disclosed in theJapanese Laid-Open Patent Application 2001-216676.

Further, with regard to the technology that achieves three-generationcompatibility, there is proposed the use of a three-step apertureswitching element in Japanese Laid-Open Patent Application 2000-187870and Japanese Laid-Open Patent Application 2003-67972.

With regard to such a conventional technology, the phase compensatingelement disclosed in the foregoing ISOM Abstract, op. cit, or JapaneseLaid-Open Patent Application has to be driven integrally with theobjective lens in view of the possibility of comma aberration when thereis caused a displacement between the objective lens and the opticalaxis. With regard to the actuator that drives the objective lens in thefocusing direction and the track direction, there is a demand ofreducing the number of the parts in order to achieve weight reductionand reduction of the number of assembling steps. In order to suppressformation of comma aberration, there is a need for the adjustment ofalignment between the objective lens and the phase compensating element.

Further, according to the method described in the foregoing ISOMAbstract, op cit., no satisfactory wave front performance is obtainedwhen to achieve the compatibility between the DVD optical recordingmedium and the CD optical recording medium.

Generally, the reference value of 0.07 λ rms, known as Marshalcriterion, is used as the wavefront aberration for the case of thediffraction limit. In the case of an optical pickup, on the other hand,there exist various factors that may become the cause of the error suchas the thickness error of the optical recording medium, the tilt errorof the optical recording medium, the defocus error caused bydisplacement between the optical recording medium and the objectivelens. Thus, it is preferred that the wavelength aberration (medianvalue) is suppressed to 0.03 λrms or less in the state there is causedno error, in view of the fact that there may be caused deterioration ofthe wavefront aberration as a result of buildup of these errors.

Contrary to this, it should be noted that the wavefront aberration(median value) becomes about 0.05 λ rms for a DVD optical recordingmedium in the technology described in the foregoing ISOM Abstract, op.cit. in view of the situation that it is not possible to minimize thewavefront aberration for both of DVD and CD by using a single element.Because of this, it was inevitable to make the design to target theintermediate point of phase compensation condition when to minimize theaberration in both of DVD and CD, while such an approach could notreduce the aberration sufficiently.

The same applies also to the numerical aperture switching elements. Inthe case of the numerical aperture switching element, it is preferablethat it is driven integrally with the objective lens in view of the factthat the desired numerical aperture value is no longer attained when theoptical axis of the objective lens is offset.

Generally, a numerical aperture switching element is mounted on anactuator such that the numerical aperture switching element is drivenintegrally with an objective lens so that the change of the numericalaperture value is avoided. On the other hand, an actuator is alsorequired to include smaller number of parts for weight reduction andreduction of the number of the assembling steps. Thus, it is desiredthat such a numerical aperture switching element is removed from theactuator. Particularly, the numerical aperture switching elementdesigned for three-generation optical recording and playback so as tooperate in three steps has a complex structure, and design andmanufacture thereof becomes difficult. Further, in spit of its complexconstruction, such a three-step numerical aperture switching elementcannot provide sufficient compensation with regard to the wavefrontcharacteristics and transmission characteristics.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean objective lens, an optical pickup and an optical informationprocessing apparatus that uses the same wherein the foregoing problemsare eliminated.

Another and more specific object of the present invention is to achievea two-generation compatibility between optical recording technology ofdifferent generations such as the blue optical recording and the DVD,without providing an aperture switching element and is simultaneouslycapable of simplifying the assembling process and achieving weightreduction of the actuator or reduction of the number of parts of theactuator.

Another object of the present invention is to achieve a three-generationcompatibility between the optical recording technology of threedifferent generations such as the blue optical recording, DVD and CD,while using a two-step numerical aperture switching element and theconventional art of the aperture switching element.

Another object of the present invention is to provide an opticalcomponent and an optical system capable of providing sufficientcompensation of spherical aberration that arises at the time ofachieving compatibility between different generations of opticalrecording technology.

Another object of the present invention is to provide an objective lensand an optical pickup sufficiently suppressing spherical aberrationbetween two generations of optical recording technology such as the blueoptical recording technology and DVD technology, or between threegenerations of optical recording technology such as the blue opticalrecording technology, DVD technology and CD technology, without the needof providing an aberration compensating element.

Another object of the present invention is to provide an objective lensand an optical pickup capable of reducing the cost and also the numberof manufacturing steps by decreasing the number of the parts usedtherein, without the need of providing an aberration compensatingelement and an aperture switching element.

According to a first aspect of the present invention, there is providedan optical pickup carrying out at least one of recording, playback anderasing of information to and from an optical recording medium,comprising:

a first optical source producing a first optical beam having awavelength λ1;

a second optical source producing a second optical beam having awavelength λ2 (λ1<λ2);

a single objective lens focusing any of said first and second opticalbeams to an optical recording medium,

said first optical beam being produced in response to turning-on of saidfirst optical source and being incident to said objective lens in thestate of a parallel beam via an infinite optical system,

said second optical beam being produced in response to turning-on ofsaid second optical source and being incident to said objective lens inthe state of a divergent optical beam via a finite optical system,

an aperture being provided at a front side of said objective lensopposite to said optical recording medium with offset from a principalpoint of said objective lens at said front side by a distance t given byan equationt=L−NA1·f/tan(a sin(NA2obj)),wherein f represents a focal distance of said objective lens, NA1represents a numerical aperture at a side of image surface in the statewhen said first optical source is turned on, NA2obj represents anumerical aperture at said front side in the state when said secondoptical source is turned on, and L represents an object distance at thetime when said second optical source is turned on.

According to the present invention, it becomes possible to achieve thetwo-generation compatibility between the optical recording technology oftwo, different generations, such as the blue optical recordingtechnology and DVD.

According to a second aspect of the present invention, there is providedan optical pickup carrying out at least one of recording, playback anderasing of information with regard to an optical recording medium,comprising:

a first optical source producing a first optical beam having awavelength λ1;

a second optical source producing a second optical beam having awavelength λ2;

a third optical source producing a third optical beam having awavelength λ3 (λ1<λ2<λ3); and

a single objective lens focusing any one of said first through thirdoptical beams upon an optical recording medium,

said first optical beam being produced in response to activation of saidfirst optical source and being incident to said objective lens in theform of a parallel beam via an infinite optical system,

said second and third optical beams being produced respectively inresponse to activation of said second and third optical sources, saidsecond and third optical beams being incident to said objective lens inthe state of a divergent beam via a finite optical system,

an aperture switching element providing an optical beam diameter φ1 whenany of said first and second optical beam is injected therethrough tosaid objective lens and an optical beam diameter φ2 (φ1>φ2) when saidthird optical beam is injected therethrough to said objective lens, saidaperture switching element being provided at a front side of saidobjective lens opposite to a side of said optical recording medium froma principal point of said objective lens at said front side with adistance offset t given by an equationt=L−NA1·f/tan(a sin(NA2obj))where f is a focal distance of said objective lens, NA1 is a numericalaperture value at the side of an image surface in the state when saidfirst optical source is turned on, NA1 obj is a numerical aperture valueat the side of said optical source in the state when said second opticalsource is turned on, and L is an object distance for the state in whichsaid second optical source is turned on.

According to the present invention, there is provided an optical pickupcapable of achieving compatibility for optical recording technology ofthree generations such as blue optical recording technology, DVD and CD,while using a two-step numerical aperture switching element, not athree-step numerical aperture element.

According to a third aspect, there is provided an optical pickupaccording to said second aspect of the present invention, wherein saidaperture switching element carries out said switching of optical beamdiameter between φ1 and φ2 by way of any of reflection, diffraction orabsorption, depending on the wavelength band or polarization direction.According to the third aspect, it becomes possible to provide an opticalpickup capable of achieving compatibility for optical recording of threegenerations such as blue optical recording technology, DVD and CD whileusing a two-step numerical aperture switching element, not a three-stepnumerical aperture element.

According to a fourth aspect of the present invention, there is providedan optical pickup according to any of the first through third aspectsnoted above, wherein there is provided a correction element thatprovides a predetermined spherical aberration to said second opticalbeam upon turning on of said second optical source, wherein saidcorrection element induces said spherical aberration with opposite phaseat the time when said second optical beam is incident to said objectivelens. Thereby, the spherical aberration caused with the optical beamupon irradiation of said optical recording medium is successfullycompensated for.

According to a fifth aspect of the present invention, there is providedan optical pickup according to said fourth aspect, wherein saidcorrection element has a concentric pattern in a surface perpendicularto an optical axis, said correction element further having across-sectional from taken in the direction parallel to said opticalaxis such that said cross-sectional form is optimized for a particularwavelength.

According to a sixth aspect of the present invention, there is providedan optical pickup according to said fourth aspect, wherein saidcorrection element has a phase pattern concentric in a surfaceperpendicular to an optical axis, said correction element further havinga stepped phase pattern in a cross-section taken in the directionparallel to said optical axis.

According to a seventh aspect of the present invention, there isprovided an optical pickup according to said fourth aspect, wherein saidcorrection element comprises a liquid crystal element inducing a phasechange in an optical beam passing therethrough in response toapplication of a voltage such that there is inducted a sphericalaberration of opposite polarity in said optical beam upon incidence tosaid objective lens.

According to the present invention as set forth in any of the fifththrough seventh aspects of the present invention, it becomes possible tocompensate for the spherical aberration caused when the optical beamhits the optical recording medium.

According to an eighth aspect of the present invention, there isprovided an optical pickup according to any of said first through thirdaspects, wherein said objective lens is formed of one group/two elementconstruction in which a concave lens and a convex lens are bonded witheach other.

According to a ninth aspect of the present invention, there is providedan optical pickup according to any of said first through third aspects,wherein said objective lens has a construction in which there is formeda diffraction pattern or in the form of a stepwise phase pattern on anyof an incident surface or exit surface of said objective lens.

According to any of the eighth aspect and ninth aspect of the presentinvention, it becomes possible to provide an optical pickup that inducesa spherical aberration of opposite phase to the incident optical beamwhen it is incident to the objective lens, without the need of using acorrection element.

According to a tenth aspect of the present invention, there is providedan optical information processing apparatus capable of carrying out atleast any one of recording, playback and erasing of information to andfrom any of an optical recording medium designed for blue opticalrecording technology that uses a blue optical beam and an opticalrecording medium designed for a red optical recording technology thatuses a red optical beam while using an optical pickup, wherein saidoptical pickup according to any of said first aspect or said fourththrough ninth aspects of the present invention is used for said opticalpickup.

According to an eleventh aspect of the present invention, there isprovided an optical information processing apparatus capable of carryingout at least one of recording, playback and erasing of information toand from any of an optical recording medium designed for a blue opticalrecording technology that uses a blue optical beam, an optical recordingmedium designed for a red optical recording technology that uses a redoptical beam and an optical recording medium designed for an infraredrecording technology that uses an infrared optical beam while using thesame optical pickup, wherein an optical pickup according to any of thefirst through ninth aspects of the present invention is used for saidoptical pickup.

According to a twelfth aspect of the present invention, there isprovided an objective lens focusing any of a first optical beam having afirst wavelength λ1 and a second optical beam having a second wavelengthλ2 (λ1<λ2) incident to an incident side of said objective lens, to anoptical recording medium provided at an exit side of said objectivelens,

said objective lens having a single-lens construction and defined by anon-spherical convex surface at said incident side and said exit side,

wherein there is satisfied a condition:νd>35  (1)1.58>nd  (2)0.58nd−0.29≦R1/f≦0.62nd−0.31  (3)

where nd and νd represent respectively the refractive index and Abbenumber for a D-line, R1 represent the paraxial radius of curvature and frepresents the focal distance.

According to a thirteenth aspect of the present invention, there isprovided an objective lens according to said twelfth aspect, whereinsaid conditions (1), (2) and (3) are set for an optical beam incidentfrom an infinite distance when said optical beam has said wavelength λ1,and wherein said conditions (1), (2) and (3) are set for an optical beamincident form a finite distance when said optical beam has saidwavelength λ2.

According to a fourteenth aspect of the present invention, there isprovided an objective lens according to any of said twelfth orthirteenth aspect, wherein said optical beam incident to said objectivelens with said wavelength λ1 is an optical beam of a blue wavelengthband, and wherein said optical beam incident to said objective lens withsaid wavelength λ2 is an optical beam of a red wavelength band, andwherein said conditions (1), (2) and (3) are set for said optical beamsof said wavelengths λ1, λ2 and λ3.

According to a fifteenth aspect of the present invention, there isprovided an objective lens focusing any of a first optical beam having awavelength λ1, a second optical beam having a wavelength λ2 and a thirdoptical beam having a wavelength λ3 (λ1<λ2<λ3) incident to an incidentside of said objective lens to an optical recording medium provided atan exit side of said objective lens,

said objective lens having a single-lens construction and defined by anon-spherical convex surface at said incident side and said exit side,

wherein there is satisfied a condition:νd>35  (1)1.58>nd  (2)0.58nd−0.29≦R1/f≦0.62nd−0.31  (3)

where nd and νd represent respectively the refractive index and Abbenumber for the D-line, R1 represent the paraxial radius of curvature andf represents the focal distance.

The objective lens according to the twelfth or fifteenth aspect of thepresent invention is a positive lens focusing a parallel beam incidentto said objective lens at said incident side. Because the objective lensfor the optical pickup of the present invention has a single lensconstruction and is defined by a non-spherical surface at both sides,the objective lens can be configured in the form of a convex lens ormeniscus lens, wherein the use of the convex lens is most preferable inview of the possibility of reducing the curvature at the incident sideand in view of the easiness of manufacture. Thereby, it should be notedthat the foregoing conditions (1), (2) and (3) are the conditions forfocusing the optical beam on the optical recording medium with the beststate of wavefront aberration for the case of conducting two-wavelengthoptical recording or three-wavelength optical recording.

In the case the condition (2) is not satisfied, for example, therefractive index of the objective lens becomes too small, and it becomesnecessary to increase the curvature of the lens surface at the incidentside for achieving the desired numerical aperture value. However,formation of such a lens surface with high precision is difficult andthe cost of the objective lens is increased inevitably.

When the conditions (1) and (2) for the refractive index nd and the Abbenumber νd is not satisfied, on the other hand, there arises the problemof excessively large chromatic aberration, and the compatibility betweentwo wavelengths or three wavelengths becomes difficult.

In the case where the lens surface of the objective lens at the incidentside is convex, there is caused an increase of positive refracting powerat this surface with increase of the paraxial curvature radial R1.Because the objective lens for an optical pickup has a large numericalaperture NA of about 0.65, it is necessary to increase the positiverefracting power at the lens. Thus, in the case of increasing theparaxial curvature radius R1 as noted above while maintaining a largenumerical value, it becomes necessary to increase the refractive indexof the lens material. Thus, there exists a relation ship “the refractiveindex nd of the lens increases with increase of the paraxial curvatureradius R1.

According to a sixteenth aspect of the present invention, there isprovided an objective lens according to said fourteenth aspect, whereinsaid conditions (1), (2) and (3) are set for an optical beam from aninfinite distance when said optical beam has said wavelength λ1 andwherein said conditions (1), (2) and (3) are set for an optical beamfrom a finite distance when said optical beam has any of said wavelengthλ2 or λ3.

According to a seventeenth aspect, there is provided an objective lensaccording to any of said fifteenth or sixteenth aspect, wherein saidoptical beam incident to said objective lens with said wavelength λ1 isan optical beam of a blue wavelength band, and wherein said optical beamincident to said objective lens with said wavelength λ2 is an opticalbeam of a red wavelength band, and wherein said conditions (1), (2) and(3) are set for said optical beams of said wavelengths λ1, λ2 and λ3.

Thus, in the objective lens of the present invention, an incidentoptical beam is incident to the objective lens in the form of an opticalbeam of a finite system in the case the optical beam is an optical beamof long wavelength.

In the case an objective lens designed for use in the blue wavelengthband with an incident optical beam of the infinite system is used withthe wavelength and substrate thickness of DVDs or CDs, there arises aspherical aberration because of the difference of substrate thickness(06 mm, 06 mm, 1.2 mm) and the difference of wavelength (405 nm, 660 nm,785 nm).

This spherical aberration can be compensated for by using a divergentoptical beam as the incident optical beam incident to the objective lendwhen conducting any of recording, playback or erasing to or from a DVDor CD. In other words, the objective lens is used as the lens of afinite optical system when carrying out any of recording, playback orerasing to and from a DVD or a CD.

Hereinafter, these inventions will be examined closely.

In order to form an optical spot on a recording surface with a “desiredspot size” with a predetermined wavelength through a substrate ofpredetermined thickness, it is necessary to set the “upper limit valueof the wavefront aberration permitted for formation of satisfactoryoptical spot” to be 0.04 λms, where λ represents the wavelength. Itshould be noted that this value of 0.04 λms includes the effect ofdeviation of curvature radius at the front lens surface and the rearlens surface, deviation of the thickness, deviation of the non-sphericalform, shifting of the front and rear lens surfaces, tilting of the frontand rear lens surfaces, and the like, caused by the manufacturing errorof the lens.

Thus, the appropriate median value set for the degradation of wavefrontis thought to be about 0.02 λms. Hereinafter, explanation will be madeby using the foregoing value of 0.02 λrms as the approximate mediumvalue.

Because the present invention achieves at least one of recording,playback and erasing for the three generations of blue optical recordingmedia, DVDs and CDs, there is a need of achieving the wavefrontaberration of 0.2 λms or less for each of the wavelengths.

FIGS. 30A-30C show the wavefront aberrations respectively for the caseof using an infinite optical system together with the numerical aperturevalue NA of 0.65-0.75 for a blue incident optical beam having thewavelength of 405 nm together with the optical recording medium having asubstrate thickness of 0.6 mm, the case of using a finite optical systemof DVD together with the numerical aperture value NA of 0.65 for theincident optical beam having the wavelength of 660 nm with the opticalrecording medium having a substrate thickness of 0.6 mm, and the case ofusing a finite optical system of CD together with the numerical apertureNA of 0.50 for the optical beam having the wavelength of 785 nm with theoptical recording medium having a substrate thickness of 1.2 mm, whereinthe lens material is represented in terms of the refractive index nd andthe Abbe number νd.

It should be noted that the condition that the wavefront aberrationbecomes 0.02 λrms or less for any of the blue system, DVD and CD is therange shown in FIG. 31 by hatching, wherein this range corresponds tothe foregoing conditions (1) and (2). Further, FIG. 31 shows thecompositional range of commercially available glasses by a broken line.

It should be noted from FIG. 30B that when these conditions (1) and (2)are not met, there appears an excessively large chromatic aberration dueto the different wavelength band of DVD, and the required aberration of0.02 λrms or less cannot be satisfied.

FIG. 32 shows the relationship between the paraxial curvature radius R1and the refractive index nd of the objective lens of an optical pickupthat has a lens surface of larger curvature at the incident side, underthe condition that the wavefront aberration of 0.02 λrms or less isachieved. In the relationship, it was assumed that the focal distance fis 3.05 mm and the numerical aperture value NA is 0.65. Here, it shouldbe noted that the marks ●, ▪, Δ, x, *, ∘ and + represent the Abbe numberof 65, 60, 55, 50, 45, 40 and 35, respectively.

As can be seen, the Abbe numbers fall within the region defined by thelines a and b. Because the refractive index depends not only on therefractive index nd of a D-line but also on the Abbe number νd, therelationship between the paraxial curvature radius R1 and the refractiveindex nd is not determined uniquely. Nevertheless, it is possible toachieve the wavefront aberration of 0.02 λrms or less, by satisfying thecondition (3) with regard to the paraxial curvature radius R1 and therefractive index nd determined by the range between the lines a and band simultaneously satisfying the condition of the refractive index nddetermined by the condition (1).

Thus, according to the objective lens of the twelfth through seventeenthaspect of the present invention, it becomes possible to achievecompatibility between two or three wavelengths used in the blue opticalrecording, DVDs and CDs, without using a conventional compound lens oradding phase compensation elements.

As noted before, the optimum numerical aperture value is different inconstructing an optical pickup between the blue optical recordingmedium, DVD recording medium and CD recording medium. Thus, there is aneed of changing the diameter of the incident beam to the objective lensaccording to the type of the optical recording medium used.

Thus, according to an eighteenth aspect of the present invention, thereis provided an optical pickup comprising:

a first optical source producing a first optical beam having a firstwavelength λ1;

a second optical source producing a second optical beam having a secondwavelength λ2; and

an objective lens according to any of the twelfth, thirteenth orfourteenth aspect of the present invention, said objective lens focusingany of said first and second optical beams on an optical recordingmedium,

said objective lens having a first effective numerical aperture valueNA(λ1) for said first optical beam and a second effective numericalaperture value NA(λ2) for said second optical beam,

said first optical beam incident to said objective lens having a firstbeam diameter φ1,

said second optical beam incident to said objective lens having a secondbeam diameter φ2,

said first and second effective numerical aperture values NA(λ1) andNA(λ2) and said first and second beam diameters satisfying arelationshipNA(λ1)=NA(λ2)  (4)φ2>φ1.  (5)

Generally, the effective numerical aperture value NA is represented byNA=φ/2/fwhere φ represents the diameter of the optical beam incident to theobjective lens, while f represents the focal distance.

The focal distance increases with increasing wavelength. Thus, bysatisfying the conditions (4) and (5) noted above, it becomes possibleto achieve the compatibility between the blue optical recordingtechnology known as HD-DVD that uses the numerical aperture value NA of0.65 and the DVD optical recording technology that uses the numericalaperture value NA of 0.65.

According to a twentieth aspect of the present invention, there isprovided an optical pickup comprising:

a first optical source producing a first optical beam having a firstwavelength λ1;

a second optical source producing a second optical beam having a secondwavelength λ2; and

an objective lens according to any of the twelfth, thirteenth orfourteenth aspect of the present invention, said objective lens focusingany of said first and second optical beams on an optical recordingmedium,

said objective lens having a first effective numerical aperture valueNA(λ1) for said first optical beam and a second effective numericalaperture value NA(λ2) for said second optical beam,

said first optical beam incident to said objective lens having a firstbeam diameter φ1,

said second optical beam incident to said objective lens having a secondbeam diameter φ2,

said first and second effective numerical aperture values NA(λ1), andNA(λ2) and said first and second beam diameters satisfying arelationshipNA(λ1)>NA(λ2)  (6)φ2=φ1.  (7)

For example, by setting the numerical aperture value NA for the bluebeam to 0.67, which is slightly larger than the numerical aperture valuefor the DVD technology, it becomes possible to use the same beamdiameter (φ2=φ1) for the incident optical beams, and the switching ofthe numerical aperture can be omitted.

Generally, the degradation of wavefront caused by the thickness error ofthe transparent substrate forming an optical recording medium increaseswith fourth power of NA, while the degradation of wavefront caused bythe tilt error of the transparent substrate increases with third powerof NA. Thus, increase of capacity cannot be achieved simply byincreasing NA.

On the other hand, in the case the degree of degradation of wavefront issuch that the use of the numerical aperture value NA of 0.65 for HD-DVDis allowed also in the technology that uses the numerical aperture valueNA of about 0.67, the degradation of such a margin is sufficiently smalland the use of the numerical aperture value NA of 0.65 is possiblewithout problem.

According to a twenty second aspect of the present invention, there isprovided an optical pickup comprising:

a first optical source producing a first optical beam having a firstwavelength λ1;

a second optical source producing a second optical beam having a secondwavelength λ2;

a third optical source producing a third optical beam having a thirdwavelength λ3; and

an objective lens according to any of the fourteenth, fifteenth orsixteenth aspect of the present invention, said objective lens focusingany of said first, second and third optical beams on an opticalrecording medium,

said objective lens having a first effective numerical aperture valueNA(λ1) for said first optical beam, a second effective numericalaperture value NA(λ2) for said second optical beam, and a thirdeffective numerical aperture value NA(λ3) for said third optical beam,

said first optical beam incident to said objective lens having a firstbeam diameter φ1,

said second optical beam incident to said objective lens having a secondbeam diameter φ2,

said third optical beam incident to said objective lens having a thirdbeam diameter φ3,

said first, second and third effective numerical aperture values NA(λ1),NA(λ2), NA(λ3) and said first, second and third beam diameters φ2, φ1,φ3 satisfying a relationshipNA(λ1)=NA(λ2)>NA(λ3)  (8)φ2>φ1>φ3.  (9)

With this, it becomes possible to achieve the optimum numerical aperturevalues NA for each of the generation of the blue optical recordingtechnology, DVD optical recording technology and CD optical recordingtechnology.

According to a twenty third aspect of the present invention, there isprovided an optical pickup comprising:

a first optical source producing a first optical beam having a firstwavelength λ1;

a second optical source producing a second optical beam having a secondwavelength λ2;

a third optical source producing a third optical beam having a thirdwavelength λ3; and

an objective lens according to any of the fourteenth, fifteenth orsixteenth aspect of the present invention, said objective lens focusingany of said first, second and third optical beams on an opticalrecording medium,

said objective lens having a first effective numerical aperture valueNA(λ1) for said first optical beam, a second effective numericalaperture value NA(λ2) for said second optical beam, and a thirdeffective numerical aperture value NA(λ3) for said third optical beam,

said first optical beam incident to said objective lens having a firstbeam diameter φ1,

said second optical beam incident to said objective lens having a secondbeam diameter φ2,

said third optical beam incident to said objective lens having a thirdbeam diameter φ3,

said first, second and third effective numerical aperture values NA(λ1),NA(λ2), NA(λ3) and said first, second and third beam diameters φ1, φ2,φ3 satisfying a relationshipNA(λ1)>NA(λ2)>NA(λ3)  (10)φ2>φ1>φ3.  (11)

With this, it becomes possible to achieve the optimum numerical aperturevalues NA for each of the generation of the blue optical recordingtechnology, DVD optical recording technology and CD optical recordingtechnology.

According to a twenty fourth aspect of the present invention, there isprovided an optical pickup comprising:

a first optical source producing a first optical beam having a firstwavelength λ1;

a second optical source producing a second optical beam having a secondwavelength λ2;

a third optical source producing a third optical beam having a thirdwavelength λ3; and

an objective lens according to any of the fourteenth, fifteenth orsixteenth aspect of the present invention, said objective lens focusingany of said first, second and third optical beams on an opticalrecording medium,

said objective lens having a first effective numerical aperture valueNA(λ1) for said first optical beam, a second effective numericalaperture value NA(λ2) for said second optical beam, and a thirdeffective numerical aperture value NA(λ3) for said third optical beam,

said first optical beam incident to said objective lens having a firstbeam diameter φ1,

said second optical beam incident to said objective lens having a secondbeam diameter φ2,

said third optical beam incident to said objective lens having a thirdbeam diameter φ3,

said first, second and third effective numerical aperture values NA(λ1),NA(λ2), NA(λ3) and said first, second and third beam diameters φ1, φ2,φ3 satisfying a relationshipNA(λ1)>NA(λ2)>NA(λ3)  (12)φ1=φ2>φ3  (13)

With this, it becomes possible to achieve the optimum numerical aperturevalues NA for each of the generation of the blue optical recordingtechnology, DVD optical recording technology and CD optical recordingtechnology.

It should be noted that beam diameter of the optical pickup according toany of the sixteenth through twenty sixth aspects of the presentinvention is determined by the aperture or a wavelength-selectiveaperture switching means moving integrally with the objective lens.

Thus, according to a twenty sixth aspect of the present invention, thereis provided an optical pickup according to any of the eighteenth throughtwentieth aspect of the present invention, wherein there is provided anaperture having a diameter generally identical with a maximum diameterof the incident optical beam, said aperture being movable integrallywith said objective lens.

According to a twenty-sixth aspect of the present invention, there isprovided an optical pickup according to any of said eighteenth, twentyfirst, twenty second and twenty third aspects of the present invention,wherein there is provided an aperture having a diameter generallyidentical with a maximum diameter of the incident optical beam and awavelength-selective aperture switching means, said aperture and saidwavelength-selective aperture switching means being movable integrallywith said objective lens.

According to a twenty seventh aspect of the present invention, there isprovided an optical pickup according to any of the nineteenth or twentythird aspects of the present invention, wherein said first optical beamis incident to said objective lens via an infinite optical system whensaid first optical source is turned on, said second optical beam isincident to said objective lens via a finite optical system when saidsecond optical source is turned on,

wherein there is provided an aperture having a diameter generally equalto said first diameter of said first optical beam incident to saidobjective lens at a predetermined distance tt≈L−NA1·f/tan(a sin(NA2obj))as measured from a principal point of said objective lens located at anincident-side lens surface in the direction opposite to an exist-sidelens surface, wherein f represents a focal distance of said objectivelens, NA1 represents a numerical aperture at a side of image surface inthe state when said first optical source is turned on, NA2obj representsa numerical aperture at said front side in the state when said secondoptical source is turned on, and L represents an object distance at thetime when said second optical source is turned on.

According to the present invention, it becomes possible to provideoptimum numerical aperture values for two generations by using theaperture only (no aperture switching means). Further, it becomespossible to provide optimum numerical aperture values for threegenerations by merely using the aperture and one step switching (nottwo-step switching).

According to a twenty eighth aspect of the present invention, there isprovided an optical pickup according to the twenty sixth aspect, whereinthe aperture switching means comprises a wavelength-selective coating ora wavelength-selective diffraction grating and is formed on any of theincident-side surface or exit-side surface of said objective lens.

Thus, it becomes possible to eliminate a dedicated element for apertureswitching and the weight of the actuator is reduced. Thereby, the numberof steps for assembling the actuator is reduced.

According to a twenty-ninth aspect of the present invention, there isprovided an optical information processing apparatus that carries out atleast one of recording, playback and erasing of information to and froman optical recording medium while using the optical pickup according toany of the twelfth through seventeenth aspect or eighteenth throughtwenty eighth aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing the construction of an optical pickupcapable of achieving compatibility between the large-capacity opticalrecording technology that uses a blue wavelength band and a conventionalDVD recording technology, while FIG. 1B is a diagram showing theconstruction of an optical pickup capable of achieving compatibilitybetween the large-capacity optical recording technology that uses a bluewavelength band and a conventional DVD or CD recording technology;

FIG. 2 is a diagram showing an exemplary construction of a conventionalobjective lens;

FIG. 3 is a diagram showing the relationship between the wavelength of aconventional optical source and the wave-front aberrationcharacteristics;

FIGS. 4A-4C are characteristic diagrams showing the distribution ofwave-front aberration for various refractive index values and variousthe Abbe numbers for the case of a blue optical recording medium, DVDmedium and CD medium;

FIG. 5 is a characteristic diagram showing the constraint imposed on theglass composition from the relationship between the Abbe number and therefractive index;

FIG. 6 is the characteristic diagram showing the relationship betweenthe refractive index and a first surface curvature radius;

FIG. 7 is a diagram showing the outline construction of an opticalpickup according to a first embodiment of the present invention;

FIG. 8 is a diagram showing the construction of a hologram unit in whicha laser diode, a hologram and a photodetection device are integrated;

FIG. 9 is a diagram showing the relationship between the diameter of theincident optical beam and the distance between the aperture and theobjective lens and further the relation ship between the diameter of theincident optical beam and the numerical aperture value;

FIG. 10 is a diagram showing the focusing action of an objective lensfocusing an incident optical beam for the case of an infinite opticalsystem and a finite optical system;

FIG. 11A is a diagram showing focusing of an incident optical beamhaving a wavelength of 407 nm by an objective lens and a phasecompensation element, while FIG. 11B shows the spherical aberrationcaused in the construction of FIG. 11A for the optical beam of the 407nm wavelength;

FIG. 12A is a diagram showing focusing of an incident optical beamhaving a wavelength of 660 nm by an objective lens and a phasecompensation element;

FIG. 12B is a diagram showing the spherical aberration caused in anoptical beam of a 660 nm wavelength incident to the objective lens thatis designed so as to minimize the aberration for the optical beam havingthe wavelength of 407 nm, via an infinite optical system;

FIG. 12C is a diagram showing the spherical aberration for the case anoptical beam of a 660 nm wavelength has been incident to the objectivelens via a finite optical system;

FIG. 12D is a diagram showing the spherical aberration caused in theoptical beam of the 660 nm wavelength for the case the optical beam isincident via a finite optical system together with phase compensation;

FIG. 13 is a diagram showing the relationship between the objectdistance and the wavefront aberration for the case of a finite DVDoptical system;

FIGS. 14A and 14B are respectively a plan view and a cross-sectionalview showing the construction of a phase compensation element accordingto Example 1 of the first embodiment of the present invention;

FIG. 15 is a diagram showing the general construction of an opticalpickup according to Example 2 of the first embodiment of the presentinvention;

FIG. 16 is a cross-sectional diagram showing the construction of thephase compensation element according to Example 2 of the firstembodiment of the present invention;

FIG. 17 is a diagram showing the general construction of an opticalpickup according to Example 3 of the first embodiment of the presentinvention;

FIG. 18 is a front view diagram showing an electrode pattern used in aliquid crystal device used for the phase compensation element of Example3;

FIG. 19 is a diagram showing the spherical aberration caused as a resultof difference of the wavelength of the used optical beam;

FIG. 20A is a diagram showing a wavefront of the spherical aberration(continuous line) and a phase shift pattern (broken line) in the form ofa two-dimensional curve;

FIG. 20B is a diagram showing the wavefront after the correction of theaberration has been made;

FIG. 21 is a diagram showing the general construction of an opticalpickup according to Example 4 of the first embodiment of the presentinvention;

FIG. 22A is a diagram showing the focusing action of an optical beam byan optical system including a phase compensation element for thewavelength of 407 nm and an objective lens having a one group/twoelement construction;

FIG. 22B is a diagram showing the spherical aberration caused in theoptical system of FIG. 21A at the wavelength of 407 nm;

FIG. 23A is a diagram showing the focusing action of an optical beam byan optical system including a phase compensation element for thewavelength of 660 nm and an objective lens having a one group/twoelement construction;

FIG. 23B is a diagram showing the spherical aberration caused in theoptical system of FIG. 23A at the wavelength of 660 nm;

FIG. 24 is a diagram showing the general construction of an opticalpickup according to Example 5 of the first embodiment of the presentinvention;

FIG. 25 is a diagram showing the construction of an objective lenscarrying a phase pattern;

FIG. 26 is a diagram showing the general construction of an opticalpickup according to a second embodiment of the present invention;

FIGS. 27A-27C are diagrams showing the construction of an apertureswitching element that changes the beam diameter of an incident opticalbeam in response the wavelength of the optical beam by causingreflection according to a second embodiment of the present invention;

FIGS. 28A-28C are diagrams showing the construction of an apertureswitching element that changes the beam diameter of an incident opticalbeam in response the wavelength of the optical beam by causingdiffraction according to a second embodiment of the present invention;

FIGS. 29A-29C are diagrams showing the construction of an apertureswitching element that changes the beam diameter of an incident opticalbeam in response the wavelength of the optical beam by causingabsorption according to a second embodiment of the present invention;

FIG. 30A is a diagram showing the focusing action of an optical systemincluding an aperture switching element for the wavelength of 780 nm andan objective lens, while FIG. 30B shows the aberration caused in theoptical system of FIG. 30A at the wavelength of 780 nm;

FIG. 31A is a diagram showing the focusing action of an objective lenshaving the one group/two element construction, while FIG. 31B shows thespherical aberration caused in the optical system of FIG. 31A at thewavelength of 780 nm;

FIG. 32 is an oblique view diagram showing the general construction ofan optical recording and playback apparatus according to a thirdembodiment of the present invention;

FIGS. 33A-33C are diagrams showing an objective lens according to afourth embodiment of the present invention;

FIG. 34 is a characteristic diagram showing the wave-front aberration ofthe objective lens of the fourth embodiment;

FIGS. 35A-35C are diagrams showing an objective lens according to afifth embodiment of the present invention;

FIG. 36 is a characteristic diagram showing the wave-front aberration ofthe objective lens of the fifth embodiment;

FIGS. 37A-37C are diagrams showing an objective lens according to asixth embodiment of the present invention;

FIG. 38 is a characteristic diagram showing the wave-front aberration ofthe objective lens of the sixth embodiment;

FIG. 39 is a diagram showing an optical system constructing an opticalpickup according to a seventh embodiment of the present invention;

FIG. 40 is a diagram showing the relationship between the wavelength andthe effective diameter for achieving the numerical aperture of 0.65;

FIGS. 41A-41C are diagrams showing the construction of switching theaperture of the optical pickup according to the seventh embodiment;

FIGS. 42A-42C are diagrams showing the construction of switching theaperture of the optical pickup according to an eighth embodiment;

FIG. 43 is a diagram showing the relationship between the numericalaperture value for the blue wavelength band and the numerical aperturevalue for the red wavelength band;

FIGS. 44A-44C are diagrams showing the schematic construction ofnumerical aperture switching of an optical pickup according to a ninthembodiment of the present invention;

FIG. 45 is a characteristic diagram showing the relationship between thediameter of the optical beam incident to an objective lens and thedistance of the aperture from the objective lens;

FIG. 46 is a diagram showing the principle of the calculation forcalculating the aperture position;

FIG. 47 is a characteristic diagram showing the relationship between thewavefront aberration and the object distance for the case of theobjective lens of which wavelength is 660 nm;

FIG. 48A-48C are schematic diagram showing an example of the coatingapplied to an objective lens; and

FIG. 49 is a schematic oblique view diagram showing an opticalinformation processing apparatus according to a tenth embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 7 is a diagram showing the schematic construction of an opticalpickup according to a first embodiment of the present invention capableof carrying out any of recording, playback and erasing of information toand from any of: a blue optical recording medium having a substratethickness of 0.6 mm at the side where an optical irradiation is made anddesigned for use with an optical source having a wavelength of 407 nmand an optical system having a numerical aperture value NA of 0.67; anda DVD optical recording medium having a substrate thickness of 0.6 mm atthe side where an optical irradiation is made and designed for use withan optical source having a wavelength of 660 nm and an optical systemhaving a numerical aperture value NA of 0.65.

While the technology of the blue generation optical recording technologyaddressed in the first embodiment of the present invention is still inthe process of consolidation, the present embodiment will be explainedfor an exemplar construction that uses the wavelength of 407 nm and thenumerical aperture value NA of 0.67. By satisfying these conditions, acapacity of about 2.7 times as large as the capacity of 4.7 GB of theconventional DVD optical recording medium is achieved.

Referring to FIG. 7, the optical pickup according to the firstembodiment of the present invention comprises: a blue optical systemformed of a laser diode 101 producing an optical beam of a bluewavelength band, a collimator lens 102, a polarization optical beamsplitter 103, a dichroic prism 204, a deflection prism 104, aquarter-wavelength plate 106, a single aperture 107, an objective lens108, a detection lens 110, an optical beam splitter 111 and aphotodetection device 112; and a DVD optical system formed of a hologramunit 201, a coupling lens 202, a phase compensation element 203 a, adichroic prism 204, the deflection prism 104, the quarter-wavelengthplate 106, the single aperture 107 and the objective lens 108, whereinthe dichroic prism 204, the deflection prism 104, the quarter-wavelengthplate 106, the single aperture 107, and the objective lens 108 are usedcommonly by the foregoing two optical system.

Here, it should be noted that the objective lens 108 is designed so asto minimize the spherical aberration when it is used with a blue opticalrecording medium having a substrate thickness of 0.6 mm at the sidethereof where irradiation is made by a blue optical beam having thewavelength of 407 nm, provided that the blue optical beam is incident tothe objective lens 108 in the form of a parallel beam by way of theoptical system forming an infinite optical system.

Further, it should be noted that there are provided optical recordingmedia 109 a and 109 b designed for different wavelengths, wherein theoptical recording medium 109 a represents the blue optical recordingmedium having the substrate thickness of 0.6 mm, while the opticalrecording medium 109 b represents the DVD optical recording mediumhaving the substrate thickness of 0.6 mm. At the time of recording orplayback any one of the optical recording media is mounted to a rotatingmechanism not illustrated that rotates the optical recording mediummounted thereon at a high speed.

It should be noted that the single aperture 107 restricting the incidentoptical beam may be provided on a bobbin, which in turn is provided onan actuator that holds the objective lens 108 in such a manner that theobjective lens 108 is moved in the focusing direction or trackingdirection, and thus, there is no need of providing a separate opticalcomponent for this purpose.

First, explanation will be made for the case of carrying out recording,playback and erasing of information to or from a blue optical recordingmedium having the substrate thickness of 0.6 mm at the side where theirradiation by the optical beam having the wavelength of 407 nm is madewhile using the optical system together with the numerical aperturevalue NA of 0.67.

The optical beam emitted from the laser diode 101 oscillating at thewavelength of 407 nm in the form of a divergent beam having a linearpolarization is shaped by the collimator lens 102 in the form of aparallel optical beam, wherein the parallel optical beam thus formed ispassed through the polarization beam splitter 103 and the dichroic prism204 disposed along an optical path thereof consecutively, wherein theoptical path of the optical beam thus passed through the polarizationbeam splitter 103 and the dichroic prism 204 is deflected by 90 degreesby the polarization prism 104. Thereafter, the optical beam is passedthrough the quarter-wavelength plate 106 and is converted to a circularpolarization beam upon passage therethrough. Further, the optical beamis restricted by the single aperture 107 to the numerical aperture valueNA of 0.67 and is incident to the objective lens 108, wherein theobjective lens 108 focuses the optical beam on the optical recordingmedium 109 a in the form of a minute beam spot. With this beam spot, theplayback, recording or erasing of information is achieved.

The optical beam reflected back from the optical recording medium 109 aforms a circular polarized beam opposite having an opposite rotatingdirection to the optical beam incident to the optical recording medium109 a and is shaped by the objective lens 108 in the form of a paralleloptical beam. The parallel optical beam thus shaped travels in theopposite direction and passes through the foregoing quarter-wavelengthplate 106 in the opposite direction. Thereby, the optical beam isconverted to a linear polarized beam having a polarization planeperpendicular to the polarization plane of the optical beam traveling tothe optical recording medium, wherein the optical beam is reflected bythe polarization optical beam splitter to the detection lens 110.Thereby, the optical beam is converged by the detection lens 110 and isdivided into plural paths leading to the photodetection element 112.Thereby, the photodetection element 112 detects an aberration signal,information signal and the servo signal from the optical beam thusfocused thereon.

Next, explanation will be made for the case of carrying out recording,playback and erasing of information to or from a DVD optical recordingmedium having the substrate thickness of 0.6 mm at the side where theirradiation by the optical beam having the wavelength of 660 nm is madewhile using the optical system together with the numerical aperturevalue NA of 0.65.

Today, a DVD pickup generally uses a hologram unit 201 having aconstruction in which a light emitting device and a photodetectiondevice are accommodated in a single. Thereby, separation of the opticalbeam is made by using a hologram.

Thus, as shown in FIG. 8, the hologram unit 201 has a construction inwhich a laser diode chip 201 a, a hologram 201 b and a photodetectiondevice 201 c are integrated.

The optical beam having the wavelength of 660 nm emitted from the laserdiode 201 a is caused to pass through the hologram 201 b and isconverted to a predetermined divergent beam of a finite optical systemby the coupling lens 202.

The optical beam thus processed is added with a predetermined sphericalaberration by the phase compensation element 203 a as will be describedlater and is reflected toward the deflection prism 104 by the dichroicprism 204, wherein the dichroic prism 204 passes an optical beam of bluewavelength band therethrough while causes reflection in an optical beamof red wavelength band.

The optical beam thus incident to the deflection prism 104 is deflectedby the angle of 90 degrees and is passed through the ¼ wavelength plate106, and the optical beam is converted to a circular polarized opticalbeam upon passage through the ¼ wavelength plate 106.

Thereafter, the optical beam is restricted to the numerical aperturevalue NA of 0.65 by the single aperture 107 and is incident to theobjective lens 108. The objective lens 108, in turn, focuses the opticalbeam upon the optical recording medium 109 b in the form of a minutebeam spot, and playback, recording or erasing of information is achievedwith this beam spot.

The optical beam reflected from the optical recording medium 109 b isthen deflected by the deflection prism 104 and is reflected by thedichroic prism 204. Further, the optical beam thus reflected isconverted to a converging beam by the coupling lens 202 and is directedto the photodetection device 201 c inside the can that also accommodatesthe laser diode 201 a by way of diffraction caused by the hologram 201b. Thereby, the photodetection device 201 c detects the aberrationsignal, information signal and the servo signal.

Here, explanation will be made on the single aperture and the aberrationcompensating element of the present invention.

The optical pickup of the present embodiment is an optical pickupachieving so-called two-generation compatibility by providing twooptical sources, one producing an optical beam in the wavelength band anthe other producing an optical beam in the red wavelength band, whereinthe optical pickup of the present embodiment does not require anaperture switching element for achieving the desired two-generationcompatibility.

FIG. 9 is a diagram showing the relationship between the incident beamdiameter (X axis) for the case an optical beam of a red wavelength bandof 660 nm is passed through an objective lens and the distance betweenthe single aperture 107 and the front side (incident side) principalpoint of the objective lens (Y axis at the left) for the case of using aglass material of BaCD5 (provided by HOYA) and the substrate thicknessof the optical recording medium of 0.6 mm. Further, the Y axis at theright of FIG. 9 represents the numerical aperture value NA for theincident optical beam diameter in the case an optical beam having awavelength of 407 nm is caused to pass through the objective lens.

In the particular case of the present embodiment in which recording andplayback is made for the blue optical recording media by using thenumerical aperture value NA of 0.67, it can be seen from FIG. 9 that theoptimum diameter φ of the incident optical beam to the objective lens is4.03 mm and the optimum distance t between the single aperture and thefront side principal point of the objective lens is 1.4 mm.

In the present embodiment, there is provided an optical pickup carryingout at least one of recording, playback and erasing of information toand from an optical recording medium, comprising: a first optical sourceproducing a first optical beam having a wavelength λ1; a second opticalsource producing a second optical beam having a wavelength λ2 (λ1<λ2); asingle objective lens focusing any of said first and second opticalbeams to an optical recording medium, said first optical beam beingproduced in response to turning-on of said first optical source andbeing incident to said objective lens in the state of a parallel beamvia an infinite optical system, said second optical beam being producedin response to turning-on of said second optical source and beingincident to said objective lens in the state of a divergent optical beamvia a finite optical system, an aperture being provided at a front sideof said objective lens opposite to said optical recording medium withoffset from a principal point of said objective lens at said front sideby a distance t given by an equationt=L−NA1·f/tan(a sin(NA2obj)),  Eq.(1.1)wherein f represents a focal distance of said objective lens, NA1represents a numerical aperture value at a side of image surface in thestate when said first optical source is turned on, NA2obj represents anumerical aperture value at said front side in the state when saidsecond optical source is turned on, and L represents an object distanceat the time when said second optical source is turned on.

With this, it becomes possible to achieve two-generation compatibilitybetween the blue optical recording technology and the DVD technology,without providing an aperture switching element.

FIG. 10 shows the optical path of the optical beam of an infinitesystem, which is produced at an infinite point and incident to anobjective lens with the wavelength λ1 and the numerical aperture valueNA1 by a one-dotted line.

On the other hand, it is known that there exists the relationshipNA1=φ/2/f  Eq.(1.2)between the focal distance f of the objective lens, the diameter φ ofthe incident optical beam and the numerical aperture value NA1.

On the other hand, in the case of the finite optical system in which theoptical beam has the wavelength λ2, the numerical aperture value NA2,the object distance L and the numerical aperture value NA2obj at theobject side is represented in FIG. 10 by the continuous line.

In the case the first aperture is provided at the distance t offset fromthe principal point of the objective lens at the front side in thedirection toward the optical source such that the beam diameter of theincident optical beam having the wavelength λ2 becomes φ, the followingrelationship is obtainedφ/2=(L−t)×tan(a sin(NA2obj)).  Eq.(1.3)

The foregoing equation (1.1) is obtained by substituting the equation(1.2) into the equation (1.3).

In the present embodiment, the construction shown in FIGS. 12A-12Dprovides the relationship φ=4.03 mm and t=1.44 mm for the optical pathcondition of λ1=407 nm and NA1=0.67 for the blue wavelength band and theoptical path condition of λ2=660 nm, Na2=0.65, L=157 mm andNA2obj=0.013.

Further, it should be noted that the glass material of the objectivelens is not limited to BaCD5 but other materials such as BaCD12,LaC1130, BaF41, NbF1, and the like, can be used. Thereby, it ispreferable to choose the glass material allowing non-spherical moldingprocess. The foregoing diameter φ of the incident optical beam or thedistance t between the single aperture and the front side principalpoint of the objective lens is affected also by the glass material usedfor the objective lens.

EXAMPLE 1

Next, an optical pickup equipped with a phase compensation element thatuses optical diffraction for the compensation element of the sphericalaberration will be explained with reference to FIG. 7 as Example 1 ofthe present invention.

In the case an optical spot is formed on a DVD recording medium 109 byusing an optical beam having a wavelength of 660 nm together with thesingle objective lens 108, which is optimized such that the wavefrontaberration becomes minimum for the optical beam having the wavelength of407 nm, by supplying the foregoing optical beam of the 660 nm wavelengthto the objective lens 108 via an infinite optical system, there arises aspherical aberration shown in FIG. 12B due to the difference in thewavelength. It should be noted that FIG. 12B shows the wavefrontaberration at the wavelength of 407 nm.

Thus, in Example 1, the optical pickup uses a finite optical system forDVD and further provides the phase compensation element 203 a in such amanner that the phase compensation element 203 a induces a sphericalaberration of opposite phase for canceling out the spherical aberrationcaused in DVD shown in FIG. 12B.

It should be noted that the use of the finite optical system means thatthe optical beam incident to the objective lens is a divergent beam orconvergent beam. Because changing of the diverging state of the opticalbeam incident to the objective lens is equivalent of changing thespherical aberration, a diverging state appropriate for reducing thespherical aberration may be used.

As shown in FIG. 13, the wavefront aberration can be suppressed when theobject distance of the DVD optical system (the distance between theoptical source and the objective lens) is changed. According to FIG. 13,it can be seen that the wavefront aberration reduced when the objectdistance is set to the range of 110-160 mm.

While FIG. 10 represents the case in which no component is interposedbetween the objective lens and the optical source, the actualconstruction of the optical pickup such as the one shown in FIG. 7disposes other component such as the coupling lens 202 therebetween,wherein the coupling lens 202 is provided for reducing the optical pathlength.

On the other hand, the use of the finite optical system alone is notsufficient for compensating for the wavefront aberration in the case theoptical system for DVD is used.

In view of the foregoing, Example 1 proposes the use of the phasecompensation element 203 a in addition to the use of the finite opticalsystem for suppressing the wavefront aberration caused in the DVDoptical path.

More specifically, the phase compensation element 203 a causes a +1thorder diffraction in the optical beam of the 660 nm wavelength andinduces thereby a spherical aberration capable of canceling out the sumof the spherical aberration in the exit optical beam of the objectivelens caused as it passes through the DVD optical recording medium havingthe thickness of 0.6 mm and the spherical aberration and the sphericalaberration of the objective lens 108. With this, the +1th orderdiffraction beam of the phase compensation element 203 a is focused uponthe recording surface of the DVD recording medium 109 b by way of theobjective lens 108 without aberration.

FIG. 14A shows the phase compensation element in a plan view while FIG.14B shows a cross sectional view of FIG. 14A.

Referring to FIG. 14A, the phase compensation element 203 a has aconcentric interference pattern and plays the role of the sphericalaberration correction mentioned before with regard to the +1thdiffraction beam.

Here, it should be noted that the phase compensation surface of Example1 is optimized for a specific wavelength. As described in “INTRODUCTIONTO DIFFRACTION OPTICAL ELEMENTS” Optical Design Study Group, OpticalSociety of Japan, (ed)., The Physical Society of Japan, not all theenergy of the incident optical beam is converted to the exit opticalbeam in a diffraction element but only a part of the incident opticalenergy is converted to the exit optical energy with a factor calleddiffraction efficiency. In the case of so-called kinoform (sawtooth)pattern, which is thought ideal in the diffraction elements, atheoretical diffraction efficiency of 100% can be achieved at a specificwavelength when it is optimized (blazed) at that wavelength. Further, adiffraction efficiency of 90% or more can be achieved even in the caseof using a diffraction grating that approximates the foregoing kinoformgrating. In Example 1, such a blazing of the phase compensation element203 b can be achieved to the DVD optical system.

Hereinafter, the actual construction and corresponding opticalperformance of the optical pickup of Example 1 will be explained.

First, the optical performance of the objective lens will be explainedfor the blue optical beam of the 407 nm wavelength while using FIG. 11Aand Table 1.

It should be noted that the objective lens 108 of Example 1 is theobjective lens designed so that the spherical aberration is minimized atthe wavelength of 407 nm. Here, it should be noted that the numericalaperture value NA is set to 0.67, the focal distance f is set to 3 mm,and the refractive index n at the wavelength of 407 nm is set to 1.6049by using BaCD5 of HOYA for the glass of the objective lens. Thenon-spherical form of the lens surface is given by a commonly knownequation (1.4) of non-spherical surface by specifying the parameters of:coordinate X in the direction of the optical axis; coordinate Y in thedirection perpendicular to the optical axis; paraxial curvature radiusR; conical constant K; and higher coefficients A, B, C, D, E, F, . . .as follows.X=(Y ² /R)/[1√{square root over ( )}{1−(1+K)Y/R ² }+AY ⁴ +BY ⁶ +CY⁸+DY¹⁰ +EY ¹² +FY ¹⁴ +GY ¹⁶ +HY ¹⁸ +JY ²⁰+ . . .   Eq. (1.4)

TABLE 1 Glass RDY(curvature THI (refractive surface radius) (thickness)index) OBJ INFINITY INFINITY STO INFINITY 1.44 S2 2.01507 1.700000BaCD5(1.6049) K: −0.674258 A: 0.364557E−02 B: 0.410494E−04; C:0.815925E−04; D: −.444548E−04 S3 −18.13584 1.656788 K: 69.056492 A:0.132534E−01; B: −.410601E−02; C: 0.595437E−03; D: −.200993E−04 S4INFINITY 0.6 PC(1.6202) IMG INFINITY 0.0 EPD: entrance pupil 4.03diameter (mm) WL: wavelength(nm) 407

In Table 1, “OBJ” represents the object point (laser diode used foroptical source). Because the objective lens 108 forms the infiniteoptical system, “INFINITY” for the curvature radius RDY and thethickness THI means that the optical source is located at the infinitedistance. Further, “STO” represents the single aperture, wherein it willbe noted that the curvature radius thereof is designated as “INFINITY”in Table 1. The thickness of the single aperture is set to zero for theconvenience of the design. Here, all the quantities having the dimensionof length is represented by “mm”.

S2 represents the lens surface of the objective lens 108 at the side ofthe optical source, while “S3” represents the lens surface of the sameobjective lens 108 at the side of the optical recording medium. Theobjective lens 108 has a thickness of 1.7 mm, and the thickness value“1.656788 mm” at the right of “curvature” in the column for S3represents the “working distance”. Further, “S4” represents the surfaceof the optical recording medium 109 a irradiated with the optical beamand hence located at the side of the optical source, while IMGrepresents the surface coincident to the recording surface. It will benoted that the distance between the surfaces S4 and IMG corresponds tothe substrate thickness at the side where the optical radiation is made,wherein the substrate thickness is 0.6 mm and the refractive index n is1.6202.

Further, EPD: entrance pupil diameter represents the beam diameter (4.03mm) of the incident optical beam, while WL: wavelength represents thewavelength (407 nm) used for the recording. In the representation of thenon-spherical coefficients, the representation such as D: −0.200993E-04means D=−0.200993×10⁻⁴.

Next, the numerals of the phase compensation element and the opticalperformance at the wavelength of 660 nm will be explained for the phasecompensation element 203 a of the DVD optical system while using FIG.12A, Table 2 and Table 3.

Referring to Tables 2 and 3, it can be seen that the shape of theobjective lens 108 is identical with that of the objective lens 108 ofFIG. 1, while the refractive index an the working distance aredifferent. Further, the phase increment caused by the increment ofoptical path length by the interference pattern formed on the phasecompensation element 203 a at the side of the objective lens 108 isrepresented asγ(h)=(C1h ² +C2h ⁴ +C3h ⁶+ . . . )×m×λ,  Eq.(1.5)wherein h represents the height from the optical axis, Ci represents theoptical path difference function coefficient of the nth (even number)order, m represents the order of the diffraction, and λ represents thewavelength. Here, the phase increment is defined such that it takes apositive value in the case the optical path length increases with regardto the optical path length on the optical axis.

Here, it should be noted that the concentric interference pattern ofExample 1 formed on the phase compensation element 203 a is designed soas to use the +1th order diffraction. It is, however, possible to usethe diffraction of any order. For example, it is possible to use asecond order diffraction.

TABLE 2 RDY Glass (curvature THI (refractive surface radius) (thickness)index) OBJ INFINITY 146.5 S1 INFINITY 0.5 FCD1(1.4951) S2(※) INFINITY 10STO INFINITY 1.44 S4 2.01507 1.700000 BaCD5(1.6049) K: −0.674258 A:0.364557E−02 B: 0.410494E−04; C: 0.815925E−04; D: −.444548E−04 S5−14.62096 1.800894 K: 69.056492 A: 0.132534E−01; B: −.410601E−02; C:0.595437E−03; D: −.200993E−04 S6 INFINITY 0.6 PC(1.5791) IMG INFINITY0.0 EPD: (mm) 4.03 WL: (nm) 660 (※)With regard to the phase correctionsurface date, reference should be made to Table 3.

In Example 1, the interference fringe pattern of the phase compensationelement 203 is designed such that the spherical aberration is minimizedat the wavelength of 660 nm, and the optical path differencecoefficients Ci are determined as represented in Table 3 below.

TABLE 3 S2 C1: −6.3005E−04; C2: 1.9631E−03; C3: −2.4763E−03; (phase C4:1.4279E−03; C5: −3.7237E−03; C3: 3.5567E−03; compensation plane)

FIG. 12D shows the wavefront aberration at the wavelength of 660 nm forthe case the phase compensation element is inserted, wherein thehorizontal axis represents the height from the optical axis while thevertical axis represents the spherical aberration. The RMS value of thespherical aberration is 0.00 λrms in this case, while this valuesatisfies the requested value of 0.030 λrms and excellent spot formationbecomes possible.

EXAMPLE 2

FIG. 15 is a diagram showing the schematic construction of the opticalpickup according to Example 2 of the present invention.

Referring to FIG. 15, Example 2 shows an optical pickup capable ofcarrying out recording, playback and erasing of information to and fromany of the blue optical recording medium by using the wavelength band of407 nm, numerical aperture value NA of 0.67 and the substrate thicknessof 0.6 mm at the side to which the optical irradiation is made and thered optical recording medium by using the wavelength band of 660 nm,numerical aperture value NA of 0.65 and the thickness of 0.6 mm at theside to which the optical irradiation is made.

The difference of the optical pickup of FIG. 15 over the optical pickupof FIG. 7 is in the point that a phase compensation element of steppedshape shown in FIG. 16 is used in place of the phase compensationelement 203 a having a blazed pitch pattern shown in FIG. 14B.Associated with this, the present example achieves the aberrationcompensation by changing the phase of the transmission light (0^(th)order light) itself, contrary to the case of Example 1 that uses the+1^(st) diffraction light.

In the case the phase compensation element 203 b is not used, thereremains a spherical aberration at the time the exit beam of the 660 nmwavelength passes through the substrate of the 0.6 mm thickness, similarto the case explained before. In Example 2, the stepped phasecompensation element 203 b is used for the means of suppressing thespherical aberration, wherein the stepped form is formed in conformitywith the residual aberration as shown in FIG. 12C.

FIG. 16 is a cross-sectional diagram of the phase compensation element203 b according to Example 2. As shown in FIG. 16, the phasecompensation element 203 b is formed by forming a concentric pattern ona glass substrate.

EXAMPLE 3

FIG. 17 is a diagram showing the schematic construction of an opticalpickup according to Example 3 of the present embodiment.

Referring to FIG. 17, the optical pickup of Example 3 is an opticalpickup capable of carrying out recording, playback or erasing ofinformation to and from any of the blue optical recording medium byusing the wavelength of 407 nm wavelength, numerical aperture value NAof 0.67 and the thickness of 0.6 nm for the substrate at the side wherethe optical irradiation is made and the DVD optical recording medium byusing the wavelength of 660 nm wavelength, numerical aperture value NAof 0.65 and the thickness of 0.6 nm for the substrate at the side wherethe optical irradiation is made.

The difference of the optical pickup of Example 3 over the opticalpickup of Example 1 or 2 is that the optical pickup of Example 2 uses adynamic phase compensation element 302 c for the compensation means ofthe spherical aberration.

For such a dynamic phase compensation element 302 c, it is possible touse an electro-optic element such as a liquid crystal device.

As shown in FIG. 18, at least one of the transparent electrodes of theliquid crystal device is patterned to form a concentric pattern, andeach of the concentric electrode patterns is driven by applying theretoa voltage independently from other concentric electrode patterns.Thereby, the refractive index n of the liquid crystal can be changedarbitrarily from n1 to n2 in the part corresponding to the concentricelectrode.

By changing the refractive index n, there is induced a phase differenceof Δn·d (2 π/λ) for the optical beam passing through each part of theliquid crystal cell, wherein Δn represents the refractive index changefor that part of the liquid crystal cell constituting the liquid crystaldevice while d represents the cell thickness.

Consider now the case in which there is induced a spherical aberrationshown in FIG. 19 as a result of the difference of wavelength of theoptical beam.

FIG. 20A shows the wavefront of the spherical aberration in the form ofa two-dimensional curve by a continuous line. When a voltage is appliedto the concentric electrodes of the liquid crystal cell such that thereis provided a phase difference shown in FIG. 20A by a broken line in theoptical beam incident to the objective lens from the optical source, itbecomes possible to cancel out the foregoing aberration of wavefront asa result of the delay caused in the wavefront of the optical beampassing through the liquid crystal device.

FIG. 20B shows the sum of the wavefront aberration shown by thecontinuous line in FIG. 20A and the delay of the wavefront caused by theliquid crystal device shown in FIG. 20A by the broken line. In otherwords, FIG. 20B shows the wavefront after the foregoing phase correctionby the liquid crystal device. As can be seen in FIG. 20B, the wavefrontaberration is reduced significantly as compared with the originalwavefront aberration shown by the continuous line in FIG. 20A.

In the phase compensation element 203 c, it is further possible tocompensate for the spherical aberration caused by a layer gap in thecase of using a two-layer optical recording medium or the assemblingerror of the optical components in the optical pickup by adjusting thevoltage applied to the liquid crystal device.

EXAMPLE 4

FIG. 21 is a diagram showing the schematic construction of an opticalpickup according to Example 4 of the present embodiment.

Referring to FIG. 21, the optical pickup of Example 4 is an opticalpickup capable of carrying out recording, playback or erasing ofinformation to and from any of the blue optical recording medium byusing the wavelength of 407 nm wavelength, numerical aperture value NAof 0.67 and the thickness of 0.6 nm for the substrate at the side wherethe optical irradiation is made and the DVD optical recording medium byusing the red wavelength of 660 nm wavelength, numerical aperture valueNA of 0.65 and the thickness of 0.6 nm for the substrate at the sidewhere the optical irradiation is made.

The difference of the optical pickup of Example 4 over the opticalpickup of Example 1, 2 or 3 s that the optical pickup of Example 4suppresses the spherical aberration at the time of recording andplayback of the DVD optical recording medium not by using the phasecompensation element but by way of optimizing the objective lens. Morespecifically, Example 4 uses an objective lens 108 b of one group/twoelement construction for the objective lens.

Generally, it is known that chromatic aberration can be reduced bybonding a lens of positive refraction power and a lens of a negativerefraction power. In Example 4, a lens capable of compensating for thechromatic aberration in the wavelength band from the blue wavelengthband to the red wavelength band is used.

Hereinafter, the actual numeric construction of the one group/twoelement objective lens used in Example 4 with reference to FIGS. 22A,22B and FIGS. 23A and 23B.

Referring to the drawings, the incident optical beam from the opticalsource (left side of FIGS. 22A and 23A) is passed through the aperture(beam diameter radius φ=4.03 mm) and is incident to the objective lens108 b, wherein the optical beam is focused upon a recording surface ofthe optical recording medium 109 after passing through the substrate towhich the optical irradiation is made. It should be noted that therecording surface is provided coincident to the right surface of theoptical recording medium 109.

FIG. 4 shows the actual data for the case of using the blue wavelengthband.

TABLE 4 RDY Glass (curvature THI (refractive surface radius) (thickness)index) OBJ INFINITY 146.5 STO INFINITY 1.44 S2 2.31274 1.847986LAC8(1.7364) K: −0.661400 A: 0.197340E−02 B: 0.756070E−04; C:0.392551E−04; D: −.986201E−05 S3 −36.95971 0.615995 EFD8(1.7317) S5−17.05615 1.232228 K: −176.103564 A: 0.859781E−02; B: −1.87435E−02; C:−.171848E−03; D: 0.783833E−04 S5 INFINITY 0.6 PC(1.6202) IMG INFINITY0.0 EPD: (mm) 4.03 WL: (nm) 407

The non-spherical surface of the lens is represented asX=(Y ² /R)/[1+√{square root over ( )}{1−(1+K)Y/R ² }+AY ⁴ +BY ⁶ +CY ⁸+DY ¹⁰ +EY ¹² +FY ¹⁴ +GY ¹⁶ +HY ¹⁸ +JY ²⁰+  Eq. (1.6)

In Table 4, “OBJ” represents the object point (laser diode used foroptical source). Because the objective lens forms the infinite opticalsystem, “INFINITY” in the columns for the curvature radius RDY and thethickness THI means that the optical source is located at the infinitedistance. Further, “STO” represents the single aperture, wherein it willbe noted that the curvature radius thereof is designated as “INFINITY”in Table 4. The thickness of the single aperture is set to zero for theconvenience of the design. Here, all the quantities having the dimensionof length is represented by “mm”.

S2 represents the lens surface of the objective lens at the side of theoptical source, while “S3” represents the surface at which the twolenses are bonded and “S4” represents the surface of the same objectivelens at the side of the optical recording medium. The separation betweenthe surfaces S2 and S4 corresponds to the thickness of the lens havingthe value of 2.463981 mm, while the thickness 1.23228 mm at the right ofthe curvature radius of the column for S4 represents the “workingdistance”.

Further, “S5 represents the surface of the optical recording medium 109irradiated with the optical beam and hence located at the side of theoptical source, while IMG represents the surface coincident to therecording surface. It will be noted that the distance between thesurfaces S5 and IMG corresponds to the substrate thickness at the sidewhere the optical radiation is made, wherein the substrate thickness is0.6 mm and the refractive index n is 1.6202. Further, EPD: entrancepupil diameter represents the beam diameter (4.03 mm) of the incidentoptical beam, while WL: wavelength represents the wavelength (407 nm)used for the recording.

Table 5 represents the actual data for the case of using the redwavelength band.

TABLE 5 RDY glass (curvature THI (refractive surface radius) (thickness)index) OBJ INFINITY 156 STO INFINITY 1.44 S2 2.31274 1.847986LAC8(1.7088) K: −0.661400 A: 0.197340E−02 B: 0.756070E−04; C:0.392551E−04; D: −.986201E−05 S3 −36.95971 0.615995 EFD8(1.6822) S5−17.05615 1.368239 K: −176.103564 A: 0.859781E−02; B: −1.87435E−02; C:−.171848E−03; D: 0.783833E−04 S5 INFINITY 0.6 PC(1.5791) IMG INFINITY0.0 EPD: (mm) 4.03 WL: (nm) 660

It should be noted that the shape of the objective lens 108 b isidentical with the case of Table 4. On the other hand, because a finiteoptical system is used, the THI of the OBJ is not infinity but is set toa finite distance of 156 mm. Further, the refractive index and workingdistance are different between the elements.

EXAMPLE 5

FIG. 21 shows the construction of an optical pickup according to Example5 of the present embodiment.

It should be noted that the optical pickup according to Example 5 isused for carrying out recording, playback and erasing of information toand from any of the blue optical recording medium having the thicknessof 0.6 mm at the side where irradiation of the optical beam is made byusing a blue optical source of 407 nm wavelength with the numericalaperture value NA of 0.67 and a DVD optical recording medium having thethickness of 0.6 mm at the side where irradiation of the optical beam ismade by using a blue optical source of 660 nm wavelength with thenumerical aperture value NA of 0.65, similarly to the case of Example 1,wherein the optical pickup of

Examples 5 is different from the optical pickup of Examples 1-4 in thepoint that the function of the phase compensation element explained withExamples 1 and 2 is integrated woth the objective lens.

Thus, in Example 5, teh spherical abrerrtion caused when the opticalbeam of the 660 nm wavelength is passed through the substrate of the DVDrecording medium having the thickness of 0.6 nm at the side where theoptical irradiation is made, is suppressed by the phase pattern formedon an objective lens 108 c as shown in FIG. 25, in place of using thephase compensation element 203 a or 203 b in combination with theobjective lens 108 as shown in FIG. 7 or FIG. 15. In the example of FIG.25, a concentric patern is formed about the optical axis on the lenssurface of the objective lens at the side of the optical source, whilethe surface on which the foregoing pattern is formed is not limited tothe lens surface at the side of the optical source. Thus, it is alsopossible to form such a concentric pattern on the lens surface at theside of the optical recording medium. Further, such a pattern may beformed on both of the lens surfaace at the side of the optical sourceand the optical recording medium.

Second Embodiment

FIG. 26 is a diagram showing the general construction of an opticalpickup capable of carrying out recording, playback and erasing ofinformation to and from any of: a blue optical recording medium having asubstrate thickness of 0.6 mm at the side where the irradiation of theoptical beam of the 407 mm wavelength is made with the numericalaperture value of 0.67; a DVD optical recording medium having asubstrate thickness of 0.6 mm at the side where the irradiation of theoptical beam of the 660 mm wavelength is made with the numericalaperture value of 0.65; and a CD optical recording medium having asubstrate thickness of 1.2 mm at the side where the irradiation of theoptical beam of the 780 mm wavelength is made with the numericalaperture value of 0.50.

Referring to FIG. 26, the optical pickup includes a blue optical systemsimilar to the one shown in FIG. 7 that includes the laser diode 101oscillating at the wavelength of 407 nm, the collimator lens 102, thepolarization beam splitter 103, the dichroic prism 204, the polarizationprism 104, the quarter-wavelength plate 106, the single aperture 107,the objective lens, the detection lens 110, the beam splitter 111 andthe photodetector 112, wherein the optical pickup of the presentembodiment further includes a dichroic prism 304 and an apertureswitching element 107 b.

Further, the optical pickup includes a DVD optical system for the 660 nmwavelength similar to the one explained in FIG. 7 including the hologramunit 201, the coupling lens 202, the phase compensation element 203 a(or 203 b or 203 c), the dichroic prisms 204 and 304, thequarter-wavelength plate 106, the single aperture 107, the apertureswitching element 107 b, and the objective lens 108.

Further, the optical pickup includes a CD optical system for the 780 nmwavelength including a hologram unit 301, a coupling lens 302, thedichroic prisms 304, the quarter-wavelength plate 106, the singleaperture 107, the aperture switching element 107 b, and the objectivelens 108.

Thus, it will be noted that the dichroic prisms 204, 304, thepolarization prism 104, the quarter wavelength plate 106, the singleaperture 107, the aperture switching element 107 b and the objectivelens 108 are used commonly by the two or three optical systems.

Here, it should be noted that the objective lens 108 is designed so thateh spherical aberration is minimized when it is used for the blueoptical recording medium having the substrate thickness of 0.6 mm at theside irradiated with the optical beam of the 407 nm for the case theoptical beam is incident to the objective lens via an infinite opticalsystem having a numerical aperture value NA of 0.65.

Further, it should be noted that the optical pickup cooperates with anyof an optical recording medium 109 a, an optical recording medium 109 band an optical recording medium 109 c designed for differentwavelengths, wherein the optical recording medium 109 a is the blueoptical recording medium having the substrate thickness of 0.6 mm, theoptical recording medium 109 b is the DVD optical recording mediumhaving the substrate thickness of 0.6 mm, and the optical recordingmedium 109 c is the CD optical recording medium having the substratethickness of 1.2 mm. As the time of recording or playback, only one ofthe optical recording media 109 a-109 c is mounted on the rotatingmechanism and rotated at high speed.

Hereinafter, explanation will be made for the case of carrying outrecording, playback and erasing of information to or from a blue opticalrecording medium having the substrate thickness of 0.6 mm at the sidewhere the irradiation by the optical beam having the wavelength of 407nm is made while using the numerical aperture value NA of 0.67 withreference to FIG. 26.

The optical beam emitted from the laser diode 101 oscillating at thewavelength of 407 nm in the form of a divergent beam having a linearpolarization is shaped by the collimator lens 102 in the form of aparallel optical beam, wherein the parallel optical beam thus formed ispassed through the polarization beam splitter 103 and the dichroicprisms 204 and 304 disposed along an optical path thereof consecutively,wherein the optical path of the optical beam thus passed through thepolarization beam splitter 103 and the dichroic prisms 204 and 304 isdeflected by 90 degrees by the polarization prism 104. Thereafter, theoptical beam is passed through the quarter-wavelength plate 106 and isconverted to a circular polarization beam upon passage therethrough.Further, the optical beam is restricted by the single aperture 107 tothe numerical aperture value NA of 0.67 and is incident to the objectivelens 108, wherein the objective lens 108 focuses the optical beam on theoptical recording medium 109 a in the form of a minute beam spot. Withthis beam spot, the playback, recording or erasing of information isachieved.

The optical beam reflected back from the optical recording medium 109 aforms a circular polarized beam opposite having an opposite rotatingdirection to the optical beam incident to the optical recording medium109 a and is shaped by the objective lens 108 in the form of a paralleloptical beam. The parallel optical beam thus shaped travels in theopposite direction and passes through the foregoing quarter-wavelengthplate 106 in the opposite direction. Thereby, the optical beam isconverted to a linear polarized beam having a polarization planeperpendicular to the polarization plane of the optical beam traveling tothe optical recording medium, wherein the optical beam is reflected bythe polarization optical beam splitter to the detection lens 110.Thereby, the optical beam is converged by the detection lens 110 and isdivided into plural paths leading to the photodetection element 112.Thereby, the photodetection element 112 detects an aberration signal,information signal and the servo signal from the optical beam thusfocused thereon.

Next, explanation will be made for the case of carrying out recording,playback and erasing of information to or from a DVD optical recordingmedium having the substrate thickness of 0.6 mm at the side where theirradiation by the optical beam having the wavelength of 660 nm is madewhile using the numerical aperture value NA of 0.65.

Today, a DVD pickup generally uses a hologram unit 201 having aconstruction in which a light emitting device and a photodetectiondevice are accommodated in a single. Thereby, separation of the opticalbeam is made by using a hologram.

Thus, as explained with reference to the first embodiment, the hologramunit 201 has a construction in which a laser diode chip 201 a, ahologram 201 b and a photodetection device 201 c are integrated.

The optical beam having the wavelength of 660 nm emitted from the laserdiode 201 a is caused to pass through the hologram 201 b and isconverted to a predetermined divergent beam of a finite optical systemby the coupling lens 202.

The optical beam thus processed is added with a predetermined sphericalaberration by the phase compensation element 203 a as will be describedlater and is reflected toward the deflection prism 104 by the dichroicprism 204, wherein the dichroic prism 204 passes an optical beam of bluewavelength band therethrough while causes reflection in an optical beamof red wavelength band.

The optical beam thus incident to the deflection prism 104 is deflectedby the angle of 90 degrees and is passed through the ¼ wavelength plate106, and the optical beam is converted to a circular polarized opticalbeam upon passage through the ¼ wavelength plate 106.

Thereafter, the optical beam is restricted to the numerical aperturevalue NA of 0.65 by the single aperture 107 and is incident to theobjective lens 108. The objective lens 108, in turn, focuses the opticalbeam upon the optical recording medium 109 b in the form of a minutebeam spot, and playback, recording or erasing of information is achievedwith this beam spot.

The optical beam reflected from the optical recording medium 109 b isthen deflected by the deflection prism 104 and is reflected by thedichroic prism 204. Further, the optical beam thus reflected isconverted to a converging beam by the coupling lens 202 and is directedto the photodetection device 201 c inside the can that also accommodatesthe laser diode 201 a by way of diffraction caused by the hologram 201b. Thereby, the photodetection device 201 c detects the aberrationsignal, information signal and the servo signal.

It should be noted that the phase compensation elements 203 a, 203 b and203 c explained with reference to Examples 1-3 of the first embodimentof the present invention is applicable alto to the present embodiment.Further, it is possible to carry out the compensation of the aberrationby the objective lens 108 b or 108 c in place of using the phasecompensation element 203 a or 203 b similarly to Examples 4 and 5 of thefirst embodiment explained with reference to FIG. 15 or 18.

Further, explanation will be made for the case of carrying outrecording, playback and erasing of information to or from a CD opticalrecording medium having the substrate thickness of 1.2 mm at the sidewhere the irradiation by the optical beam having the wavelength of 780nm is made while using the numerical aperture value NA of 0.50.

Similarly to the case of DVD, a CD pickup generally uses a hologram unit201 having a construction in which a light emitting device and aphotodetection device are accommodated in a single. Thereby, separationof the optical beam is made by using a hologram.

Thus, similarly to the hologram unit 201 explained above, there isconstructed a hologram unit 301 in which a laser diode chip 301 a, ahologram 301 b and a photodetection device 301 c are integrated.

The optical beam having the wavelength of 780 nm emitted from the laserdiode 301 a of the hologram unit 301 is caused to pass through thehologram 301 b and is converted to a predetermined divergent beam of afinite optical system by the coupling lens 302.

The optical beam thus processed is reflected toward the deflection prism104 by the dichroic prism 304, wherein the dichroic prism 304 passes anoptical beam of blue and red wavelength band therethrough while causesreflection in an optical beam of infrared wavelength band.

The optical beam thus incident to the deflection prism 104 is deflectedby the angle of 90 degrees and is passed through the ¼ wavelength plate106, and the optical beam is converted to an elliptical a circularpolarized optical beam upon passage through the ¼ wavelength plate 106.

Thereafter, the optical beam is restricted to the numerical aperturevalue NA of 0.50 by the single aperture 107 and is incident to theobjective lens 108. The objective lens 108, in turn, focuses the opticalbeam upon the optical recording medium 109 c in the form of a minutebeam spot, and playback, recording or erasing of information is achievedwith this beam spot.

The optical beam reflected from the optical recording medium 109 b isthen deflected by the deflection prism 104 and is reflected by thedichroic prism 304. Further, the optical beam thus reflected isconverted to a converging beam by the coupling lens 302 and is directedto the photodetection device 301 c. Thereby, the photodetection device301 c detects the aberration signal, information signal and the servosignal.

Thus, the optical pickup according to the second embodiment of thepresent invention is a so-called three-generation optical pickup havingthree optical sources, one for the blue wavelength band, one for the DVDwavelength band and one for the infrared wavelength band.

In the optical pickup of the present embodiment, the desiredthree-generation compatibility is achieved by providing the apertureswitching element 107 b. On the other hand, it should be noted that thepresent embodiment uses the construction that uses the same aperture forthe blue optical recording technology and the DVD technology whenachieving the foregoing three-generation compatibility. Thereby, itbecomes possible with the present embodiment to use the conventionalthree-step aperture switching technology used in the conventional DVD/CDcompatible optical system also in the three-generation compatibleoptical system. It should be noted that the aperture switching element107 b may be the one that uses any of reflection, diffraction orabsorption when carrying out the switching of the optical beam diameterin response to the wavelength band or polarization direction.

Further, according to the present embodiment, the three-generationcompatibility is achieved by an optical pickup comprising:

a first optical source producing a first optical beam with a wavelengthλ1;

a second optical source producing a second optical beam with awavelength λ2;

a third optical source producing a third optical beam with a wavelengthλ3 (λ1<X2<λ3); and

a single objective lens focusing any of said first through third opticalbeams to an optical recording medium,

said first optical beam is incident to said objective lens in the formof a parallel optical beam when said first optical source is activated,

said second optical beam and said third optical beam is incident to saidobjective lens in the form of a divergent optical beam when any of saidsecond and third optical sources is activated,

wherein there is provided an aperture switching element providing afirst beam diameter φ1 when any of said first and second optical beamsis passed therethrough and a second beam diameter φ2 (φ1>φ2) when saidthird optical beam is passed therethrough, such that said apertureswitching element is disposed at a distance t from a principal point ofsaid objective lens at a side away from said optical recording mediumgiven ast=L−NA1·f/tan(a sin(NA2obj))  Eq. (1.7)where f represents the focal distance of the objective lens, NA1represents the numerical aperture value at the side of the image planewhen the first optical source is tuned on, NA2obj represents thenumerical aperture value at the side away from the optical recordingmedium when the second optical source is turned on, and L represents theobject distance for the case the second optical source is turned on.

With this, a three-generation compatibility between the blue opticalrecording technology, DVD technology and CD technology can be achievedby using a two-step aperture switching element, not a three-stepaperture switching element.

For the aperture switching element of the present embodiment, it ispossible to use the means for switching the beam diameter depending onthe wavelength of the optical beam emitted from the optical source byreflection as represented in FIGS. 27A-27C.

More specifically, it is possible to use a dielectric optical multilayerfilm 107 bf having wavelength selectivity for this purpose.

Referring to FIGS. 27A-27C, the aperture switching element 107 b showshigh transmissivity for any of the blue wavelength band, red wavelengthband and infrared wavelength band at the central region φ2 where thedielectric optical multilayer film 107 bf is not provided. On the otherhand, the circumference region (outside of the central region φ2 showshigh transmissivity for only the optical beam of the blue wavelengthband and the red optical wavelength band while low transmissivity forthe optical beam of the infrared wavelength band.

Similarly to the first embodiment, the optical beam of the bluewavelength band or red wavelength band is restricted to have the beamdiameter of φ1 or less by the aperture (single aperture) formed on theactuator of the objective lens. Further, it is possible to restrict theoptical beam of the blue wavelength band or red wavelength band byproviding an opaque means such as rough surface on the surface of theaperture switching element.

As the aperture switching element of the present embodiment, it is alsopossible to provide the beam diameter by diffraction as shown in FIGS.28A-28C. More specifically, it is possible to provide a diffractiongrating 107 bd having a wavelength selectivity for this purpose.

Referring to FIGS. 28A-28C, the aperture switching element 107 has ahigh transmissivity for any of the blue wavelength band, red wavelengthband and infrared wavelength band at the central region having thediameter φ2 where there is formed no diffraction grating 107 bd. In theregion outside the central part φ2, the diffraction grating 107 bd doesnot work on the blue and red wavelength beam and only the red wavelengthbeam experiences diffraction.

Similarly to the first embodiment, the optical beam of the bluewavelength band and the red wavelength band is restricted to have thebeam diameter φ1 by the aperture (single aperture) formed on theobjective lens actuator.

Further, it is possible to switch the optical beam diameter according tothe wavelength of the optical beam emitted from the optical source byabsorption as shown in FIGS. 29A-29C, wherein it will be noted thatthere is provided a selective absorption element 107 ba on the apertureswitching element 107 b in the construction of FIGS. 29A-29C.

Heretofore, explanation has been made on the aperture switching elementfor switching the beam diameter depending on the wavelength, while thepresent invention is not limited to such a specific embodiment but otherconstruction may be used. For example, it is possible to use thepolarization of the optical beam. In this case, the optical source forthe red wavelength and the optical source for the infrared wavelengthare disposed so that the polarization directions cross with each other.Thereby, the switching of the aperture is achieved according to themutually perpendicular polarization directions.

Further, it is possible to provide the means for restricting the beamdiameter to φ2 in the infrared wavelength band as provided to theaperture switching element also on the objective lens surface. In thiscase, it is possible to eliminate the aperture switching elementsimilarly to the first embodiment of the present invention. As the meansfor restricting the beam diameter, it is possible to provide a coatingof thin film causing reflection or absorption as explained before.Alternatively, it is possible to provide a diffraction pattern.

Next, explanation will be made on the finite construction of the CDoptical system with reference to FIG. 30A and Table 6, wherein it shouldbe noted that Table 6 shows the numerical parameters of the CD opticalsystem and the optical performance at the wavelength of 780 nm.

Referring to Table 6, the objective lens has the same shape as that ofthe objective lens of Table 1, while it will be noted that therefractive index, working distance and the aperture diameter arechanged. Further, a finite optical system is used for the CD opticalsystem for minimizing the spherical aberration.

TABLE 6 RDY glass (curvature THI refractive surface radius) (thickness)index OBJ INFINITY 52.26 STO INFINITY 1.44 S2 2.01507 1.700000BaCD5(1.5825) K: −0.674258 A: 0.364557E−02 B: 0.410494E−04; C:0.915925E−04; D: −.4445481E−04 S3 −14.62096 1.546207 K: 69.056492 A:0.132534E−02; B: −.410601E−02; C: 0.595437E−03; D: −.200993E−04 S4INFINITY 1.2 PC(1.5728) IMG INFINITY 0.0 EPD: (mm) 3.14 WL: (nm) 780

As represented in Table 6, an optimum wavefront is achieved by settingthe object distance (the distance from OBJ to S3) to 53.7 mm. In theactual construction, reduction of the optical path length is achieved byinterposing a coupling lens 302 to the optical path between the opticalsource and the objective lens as shown in the construction of FIG. 23.

FIG. 30B shows the wavefront aberration at the wavelength of 780 nm,wherein the horizontal axis represents the height from the optical axiswhile the vertical axis represents the wavefront aberration. In thestate of FIG. 30B, the RMS value of the wavefront is 0.006 λrms, whilethis value satisfies the condition of 0.030 λrms imposed from theviewpoint of actual use. Thus, excellent spot formation is achieved.

In the present embodiment, the optimum wavefront is achieved for theoptical system of CD by using the finite construction, while the optimumwavefront is achieved also for the DVD optical path by using theaberration compensation element shown in Examples 1-5 of the firstembodiment.

It should be noted that the one group/two element construction used inExample 4 of the first embodiment is applicable also to the CD opticalpath as showing in FIG. 31A and Table 7.

TABLE 7 RDY glass (curvature THI (refractive surface radius) (thickness)index) OBJ INFINITY INFINITY STO INFINITY 0.0 S2 2.31274 1.847986LAC8(1.7040) K: −0.661400 A: 0.197340E−02; B: 0.756070E−04; C:0.392551E−04; D: −.986201E−05 S3 −36.95971 0.615995 EFD8(1.6750) S5−17.05615 1.204793 K: −176.103564 A: 0.859781E−02; B: −1.87435E−02; C:−.171848E−03; D: 0.783833E−04 S5 INFINITY 1.2 PC(1.5728) IMG INFINITY0.0 EPD: (mm) 3.14 WL: (nm) 780

Third Embodiment

FIG. 32 is a diagram showing an information recording apparatus 30realized by the optical information processing apparatus according to athird embodiment of the present invention.

Referring to FIG. 32, the information recording apparatus 30 is anapparatus carrying out at least one of recording, playback and erasingof information to and from an optical recording medium by using anoptical pickup 31.

In the present embodiment, it should be noted that the optical recordingmedium 40 is implemented in the form of a disk accommodated in acartridge 41 or protective case. Thus, the optical recording medium 40is loaded to the information recording apparatus 30 by inserting thedisk 40 into an insertion opening 32 of the information recordingapparatus 30 in the direction of an arrow together with the cartridge41. The disk 40 thus loaded is rotated by a spindle motor 33, andrecording, playback or erasing of information is achieved by the opticalpickup 31.

For such the optical pickup 31, it is possible to use the optical pickupdescribed in first or second embodiment of the present invention.Further, any of the blue optical recording medium red optical recordingmedium or infrared optical recording medium can be used for the opticaldisk 40 when carrying out any of recording, playback or erasing of theinformation.

According to the present embodiment, it becomes possible to achieve thetwo-generation compatibility between blue/DVD technologies without usingan aperture switching element. Further, it becomes possible to achievethe three-generation compatibility between blue/DVD/CD technologieswhile using the two-step aperture switching element, not the three-stepaperture switching element. With this, it becomes possible to achievemultiple-generation compatibility with simple construction. Further, byusing a static or dynamic phase compensation element or by optimizingthe construction of the objective lens, it becomes possible to secure asatisfactory beam spot performance when carrying out recording orplayback of DVD or CD. Thereby, a high S/N optical pickup compatiblewith plural generations of optical recording technologies and an opticalinformation processing apparatus using such an optical pickup areobtained.

Fourth Embodiment

Hereinafter, an objective lens according to a fourth embodiment of thepresent invention will be described with reference to FIGS. 33A-33C andFIG. 34. While the description hereinafter will be made for the case ofachieving three-generation compatibility between the blue/DVD/CDtechnologies, it is also applicable for achieving two-generationcompatibility between blue/DVD technologies or blue/CD technologies.

It should be noted that the present embodiment relates to an objectivelens used in an optical pickup carrying out any of recording, playbackand erasing to and from any of: a blue optical recording medium having asubstrate thickness of 0.6 mm and designed for use with the wavelengthλ1 of 405 nm and the numerical aperture value NA of 0.65; a DVDrecording medium having a substrate thickness of 0.6 mm and designed foruse with the wavelength λ2 of 660 nm and the numerical aperture value NAof 0.65; and a CD recording medium having a substrate thickness of 1.2mm and designed for use with the wavelength λ3 of 785 nm and thenumerical aperture value NA of 0.50.

First, the optical performance of an objective lens 2A will be explainedfor the case when it is used with the blue recording medium 1 a designedfor the wavelength λ1 of 405 nm with reference to FIG. 33A and Table 8A.The objective lens 2A of the present embodiment has the numericalaperture value NA of 0.65, the focal distance f of 3.05 mm, therefractive index nd of a D-line of 1.50 and the Abbe number νd of 60 andis formed of a glass (500,000.600000).

Further, the objective lens 2A of the present embodiment has itsnon-spherical surface described in terms of the coordinate X in theoptical axis direction, the coordinate Y in the direction perpendicularto the optical axis, the paraxial curvature radius R, conical constantK, and higher order coefficients A, B, C, D, E, F, . . . according tothe known relationship ofX=(Y ² /R)·[1+√{square root over ( )}{1−(1+K)Y/R ² }+AY ⁴ +BY ⁶ +CY⁸+DY¹⁰ +EY ¹² +FY ¹⁴ +GY ¹⁶ +HY ¹⁸ +JY ²⁰+ . . .

Table 8A shows exemplary data, wherein “OBJ” represents the object point(laser diode used for optical source). Because the objective lens 108forms the infinite optical system, “INFINITY” for the curvature radiusRDY and the thickness THI means that the optical source is located atthe infinite distance. Further, “STO” represents the incident pupilsurface of which curvature radius is designated as “INFINITY” in Table8A and the thickness thereof is set to zero for the convenience of thedesign. Here, all the quantities having the dimension of length isrepresented by “mm”.

S2 represents the lens surface of the objective lens 108 at the side ofthe optical source, while “S3” represents the lens surface of the sameobjective lens 108 at the side of the optical recording medium. Theobjective lens 108 has a thickness of 1.85 mm, and the thickness value“1.651193 mm” at the right of “curvature radius” in the column for S3represents the “working distance”. Further, “S4” represents the surfaceof the optical recording medium 109 a irradiated with the optical beamand hence located at the side of the optical source, while IMGrepresents the surface coincident to the recording surface. It will benoted that the distance between the surfaces S4 and IMG corresponds tothe substrate thickness at the side where the optical radiation is made,wherein the substrate thickness is 0.6 mm and the refractive index n is1.62.

Further, EPD: entrance pupil diameter represents the beam diameter(3.965 mm) of the incident optical beam, while WL: wavelength representsthe wavelength (405 nm) used for the recording. In the representation ofthe non-spherical coefficients, the representation such as D:0.306790E-05 means D=0.306790×10⁻⁵.

Further, it should be noted that the objective lens 2A of the presentinvention satisfies the conditionsνd>35  (2.1)1.58>nd  (2.2)0.58 nd−0.29≦R1/f≦0.62nd−0.31  (2.3)

Next, explanation will be made for the case of using the objective lenswith a DVD medium 1 b designed for the wavelength of 660 nm will beexplained with reference to FIG. 33B and Table 8B.

Referring to FIG. 33B and Table 8B, the shape of the objective lens 2Ais the same as that of Table 8A, while the refractive index and theworking distance are different. Further, the DVD medium 1 b is used witha finite optical system in which the optical beam is incident to theobjective lens in the form of a divergent beam, and thus, the distancebetween the object point OBJ (laser diode used for the optical source)and the single aperture of the first surface STO is set to 137 mm, whileit should be noted that this value is chosen for minimizing thewavelength aberration.

Next, the case of using the objective lens with the CD medium 1 cdesigned for the wavelength of 785 nm will be explained with referenceto FIG. 33C and Table 8C.

Referring to FIG. 33C and Table 8C, the shape of the objective lens 2Ais identical with the case of FIG. 33A and Table 8A, while it will benoted that the refractive index, working distance and the substratethickness at the side where the optical irradiation is made aredifferent. Further, the CD medium 1 c is used with a finite opticalsystem in which the optical beam is incident to the objective lens inthe form of a divergent beam, and the distance from the object point OBJ(laser diode used for the optical source) to the single aperture in thefirst surface STO is set to 45.0 mm. It should be noted that this valueis chosen for minimizing the wavefront aberration similarly to the caseof DVD.

TABLE 8A RDY (curvature THI glass Surface radius) (thickness)(refractive index) OBJ INFINITY INFINITY STO INFINITY 0.0 S2 1.857651.85 500000.600000(1.514) K: −0.677487 A: 0.273561E−02 B: 0.386515E−03;C: 0.536182E−04; D: 0.306790E−05 S3 −6.68963 1.651193 K: −10.002091 A:0.138585E−01; B: −.255540E−02; C: 0.277769E−03; D: −.140136E−04 S4INFINITY 0.6 PC(1.621) IMG INFINITY 0.0 EPD: (mm) 3.965 WL: (nm) 405

TABLE 8B RDY (curvature THI Glass Surface radius) (thickness)(refractive index) OBJ INFINITY 137 STO INFINITY 0.0 S2 1.85765 1.85500000.600000(1.497) K: −0.677487 A: 0.273561E−02 B: 0.386515E−03; C:0.536182E−04; D: 0.306790E−05 S3 −6.68963 1.801922 K: −10.002091 A:0.138585E−01; B: −.255540E−02; C: 0.277769E−03; D: −.140136E−04 S4INFINITY 0.6 PC(1.579) IMG INFINITY 0.0 EPD: (mm) 4.095 WL: (nm) 660

TABLE 8C RDY (curvature THI glass surface radius) (thickness)(refractive index) OBJ INFINITY 45.0 STO INFINITY 0.0 S2 1.85765 1.85500000.600000(1.494) K: −0.677487 A: 0.273561E−02 B: 0.386515E−03; C:0.536182E−04; D: 0.306790E−05 S3 −6.68963 1.651193 K: −10.002091 A:0.138585E−01; B: −.255540E−02; C: 0.277769E−03; D: −.140136E−04 S4INFINITY 1.2 PC(1.573) IMG INFINITY 0.0 EPD: (mm) 3.28 WL: (nm) 785

FIG. 34 is a characteristic diagram showing the wavefront aberrationcaused when the objective lens 2A of the present invention is used withthe blue recording medium 1 a, DVD medium 1 b and the CD medium withrespective, predetermined wavelengths, wherein the horizontal axisrepresents the optical recording media 1 a, 1 b and 1 c, while thevertical axis represents the wavefront aberration on the optical axisfor the best image point.

It will be noted from FIG. 34 that there is achieved excellent wavefrontaberration of 0.02 λrms for each of the optical recording media 1 a, 1 band 1 c.

Fifth Embodiment

Next, an objective lens 2B according to a fifth embodiment of thepresent invention will be explained with reference to FIGS. 35A-35C andFIG. 36, wherein the present embodiment uses the objective lens 2B with:the blue recording medium 1 a having the substrate thickness of 0.6 mmand designed for use with the wavelength λ1 of 405 nm and the numericalaperture value NA of 0.70; the DVD medium having the substrate thicknessof 0.6 mm and designed for use with the wavelength λ2 of 660 nm and thenumerical aperture value NA of 0.65; and the CD medium having thesubstrate thickness of 1.2 mm and designed for use with the wavelengthλ3 of 785 nm and the numerical aperture value NA of 0.50, for carryingout recording, playback and erasing to and from any of these threedifferent optical recording media.

It should be noted that the objective lens 2B of the present embodimenthas the focal distance f of 3.05 mm, the refractive index nd and theAbbe number νd of respectively 1.55 and 60 for the D-line, and the glasstype of (550000.600000). Similarly to the fourth embodiment, theobjective lens 2B of the present embodiment is used with the infiniteoptical system when used with the blue optical recording medium 1 a,while it is used with a finite optical system in the case of using theobjective lens 2B with the DVD or CD medium.

FIG. 35A and Table 9A show the construction for the case of using theobjective lens 2B with the blue optical recording medium 1 a, while FIG.35B and Table 9B show the construction for the case of using theobjective lens 2B with the DVD medium 1 b. Further, FIG. 35C and Table9C show the construction for the case of using the objective lens 2Bwith the CD medium 1 c. Here, it should be noted that the non-sphericalshape of the objective lens 2B and the items shown in Tables 9A-9C areidentical with the case of the fourth embodiment explained before.

Further, it should be noted that the objective lens 2B of the presentembodiment also satisfies the foregoing conditions ofνd>35  (2.1)1.58>nd  (2.2)0.58nd−0.29≦R1/f≦0.62nd−0.31.  (2.3)

TABLE 9A RDY (curvature THI glass surface radius) (thickness)(refractive index) OBJ INFINITY INFINITY STO INFINITY 0.0 S2 1.9547 1.85550000.600000(1.565) K: −0.679252 A: 0.311827E−02 B: 0.382688E−03; C:0.272528E−04; D: 0.200561E−05 S3 −9.67786 1.662831 K: −15.331354 A:0.128339E−01; B: −.272936E−02; C: 0.365079E−03; D: −.233260E−04 S4INFINITY 0.6 PC(1.621) IMG INFINITY 0.0 EPD: (mm) 4.271 WL: (nm) 405

TABLE 9B RDY (curvature THI glass surface radius) (thickness)(refractive index) OBJ INFINITY 153 STO INFINITY 0.0 S2 1.9547 1.85550000.600000(1.547) K: −0.679252 A: 0.311827E−02 B: 0.382688E−03; C:0.272528E−04; D: 0.200561E−05 S3 −9.67786 1.781112 K: −15.331354 A:0.128339E−01; B: −.272936E−02; C: 0.365079E−03; D: −.233260E−04 S4INFINITY 0.6 PC(1.5791) IMG INFINITY 0.0 EPD: (mm) 4.095 WL: (nm) 660

TABLE 9C RDY (curvature THI glass surface radius) (thickness)(refractive index) OBJ INFINITY 47 STO INFINITY 0.0 S2 1.9547 1.85550000.600000(1.544) K: −0.679252 A: 0.311827E−02 B: 0.382688E−03; C:0.272528E−04; D: 0.200561E−05 S3 −9.67786 1.577408 K: −15.331354 A:0.128339E−01; B: −.272936E−02; C: 0.365079E−03; D: −.233260E−04 S4INFINITY 1.2 PC(1.573) IMG INFINITY 0.0 EPD: (mm) 3.275 WL: (nm) 785

FIG. 36 is a characteristic diagram showing the wavefront aberrationcaused when the objective lens 2B of the present invention is used withthe blue recording medium 1 a, DVD medium 1 b and the CD medium withrespective, predetermined wavelengths, wherein the horizontal axisrepresents the optical recording media 1 a, 1 b and 1 c, while thevertical axis represents the wavefront aberration on the optical axisfor the best image point.

It will be noted from FIG. 34 that there is achieved excellent wavefrontaberration of 0.02 λrms for each of the optical recording media 1 a, 1 band 1 c.

Sixth Embodiment

Next, an objective lens 2C according to a sixth embodiment of thepresent invention will be explained with reference to FIGS. 37A-37C andFIG. 38, wherein the present embodiment uses the objective lens 2C with:the blue recording medium 1 a having the substrate thickness of 0.6 mmand designed for use with the wavelength λ1 of 405 nm and the numericalaperture value NA of 0.70; the DVD medium having the substrate thicknessof 0.6 mm and designed for use with the wavelength λ2 of 660 nm and thenumerical aperture value NA of 0.65; and the CD medium having thesubstrate thickness of 1.2 mm and designed for use with the wavelengthλ3 of 785 nm and the numerical aperture value NA of 0.50, for carryingout recording, playback and erasing to and from any of these threedifferent optical recording media.

It should be noted that the objective lens 2C of the present embodimenthas the focal distance f of 3.05 mm, the refractive index nd and theAbbe number νd of respectively 1.55 and 55 for the D-line, and the glasstype of (550000.600000). Similarly to the fourth embodiment, theobjective lens 2B of the present embodiment is used with the infiniteoptical system when used with the blue optical recording medium 1 a,while it is used with a finite optical system in the case of using theobjective lens 2B with the DVD or CD medium.

FIG. 37A and Table 10A show the construction for the case of using theobjective lens 2C with the blue optical recording medium 1 a, while FIG.37B and Table 10B show the construction for the case of using theobjective lens 2C with the DVD medium 1 b. Further, FIG. 37C and Table10C show the construction for the case of using the objective lens 2Cwith the CD medium 1 c. Here, it should be noted that the non-sphericalshape of the objective lens 2B and the items shown in Tables 10A-10C areidentical with the case of the fourth embodiment explained before.

Further, it should be noted that the objective lens 2C of the presentembodiment also satisfies the foregoing conditions ofνd>35  (2.1)1.58>nd  (2.2)0.58nd−0.29≦R1/f≦0.62nd−0.31.  (2.3)

TABLE 10A RDY (curvature THI glass surface radius) (thickness)(refractive index) OBJ INFINITY INFINITY STO INFINITY 0.0 S2 1.957951.85 550000.600000(1.567) K: −0.677791 A: 0.313034E−02 B: 0.374056E−03;C: 0.278874E−04; D: 0.796110E−05 S3 −9.78389 1.627965 K: −12.939440 A:0.133065E−01; B: −.290188E−02; C: 0.395558E−03; D: −.254794E−04 S4INFINITY 0.6 PC(1.621) IMG INFINITY 0.0 EPD: (mm) 4.11 WL: (nm) 405

TABLE 10B RDY (curvature THI glass surface radius) (thickness)(refractive index) OBJ INFINITY 142 STO INFINITY 0.0 S2 1.95795 1.85550000.600000(1.547) K: −0.677791 A: 0.313034E−02 B: 0.374056E−03; C:0.278874E−04; D: 0.796110E−05 S3 −9.78389 1.795218 K: −12.939440 A:0.133065E−01; B: −.290188E−02; C: 0.395558E−03; D: −.254794E−04 S4INFINITY 0.6 PC(1.5791) IMG INFINITY 0.0 EPD: (mm) 4.11 WL: (nm) 660

TABLE 10C RDY (curvature THI glass surfaec radius) (thickness)(refractive index) OBJ INFINITY 48 STO INFINITY 0.0 S2 1.95795 1.85550000.600000(1.543) K: −0.677791 A: 0.313034E−02 B: 0.374056E−03; C:0.278874E−04; D: 0.796110E−05 S3 −9.78389 1.584558 K: −12.939440 A:0.133065E−01; B: −.290188E−02; C: 0.395558E−03; D: −.254794E−04 S4INFINITY 1.2 PC(1.573) IMG INFINITY 0.0 EPD: (mm) 3.285 WL: (nm) 785

FIG. 38 is a characteristic diagram showing the wavefront aberrationcaused when the objective lens 2C of the present invention is used withthe blue recording medium 1 a, DVD medium 1 b and the CD medium withrespective, predetermined wavelengths, wherein the horizontal axisrepresents the optical recording media 1 a, 1 b and 1 c, while thevertical axis represents the wavefront aberration on the optical axisfor the best image point.

It will be noted from FIG. 38 that there is achieved excellent wavefrontaberration of 0.02 λrms for each of the optical recording media 1 a, 1 band 1 c.

Seventh Embodiment

Next, the application of an optical pickup 11 to the NA0.65 blue/DVD/CDtechnologies will be described as a seventh embodiment of the presentinvention.

First, the overall construction of the optical system that uses theoptical pickup 11 of the present embodiment will be described withreference to FIG. 39.

It should be noted that the optical pickup 11 of the present embodimentis an optical pickup having an optical source 12 of the wavelength λ1 of405 nm for the blue wavelength band, an optical source 13 of thewavelength λ2 of 660 nm for the red wavelength band, and an opticalsource 14 of the wavelength λ3 of 785 nm for the infrared wavelengthband, wherein the optical pickup 11 is capable of carrying outrecording, playback and erasing to and from any of: the blue opticalrecording medium 1 a having the substrate thickness of 0.6 mm at theside irradiated with the optical beam and designed for use with thenumerical aperture value NA of 0.65; the DVD medium having the substratethickness of 0.6 mm at the side irradiated with the optical beam anddesigned for use with the numerical aperture value NA of 0.65; and theCD medium having the substrate thickness of 1.2 mm at the sideirradiated with the optical beam and designed for use with the numericalaperture value NA of 0.50.

It should be noted that the optical pickup 11 of the present embodimentcomprises: a blue optical system 26 for the blue wavelength bandincluding a laser (optical source) 12 of the blue wavelength band, acollimator lens 15, a polarization beam splitter 16, dichroic prisms 17and 18, a deflection prism 19, a quarter wavelength plate 20, anaperture 21, an aperture switching means 22, an objective lens 2 (thelens 2A in the present embodiment), a detection lens 23, a beam splitter24 and a photodetector 25; a red optical system 29 for DVD including ahologram unit 27, a coupling lens 28, the dichroic mirrors 17 and 18,the deflection prism 19, the quarter-wavelength plate 20, the aperture21, the aperture switching means 22 and the objective lens 2A; and aninfrared optical system 32 for CD including a hologram unit 30, acoupling lens, the dichroic prism 18, the deflection prism 19, thequarter-wavelength plate 20, the aperture 21, the aperture switchingmeans 22 and the objective lens 32. Thus, the dichroic prisms 17 and 18,the deflection prism 19, the quarter-wavelength plate 20, the aperture20, the aperture switching means 22 and the objective lens 2A are usedcommonly between the two or three optical systems.

In the hologram unit 27, it should be noted that the chip of the laserdiode 13, hologram 33 and the photodetection device 34 are integrated.Further, the hologram unit 30 integrates the chip of the laser diode 14,the hologram 35 an the photodetection device 36.

Further, as noted before, the optical recording medium 1 a is a blueoptical recording medium having a substrate thickness of 0.6 mm, theoptical recording medium 1 b is a DVD medium having a substratethickness of 0.6 mm, while the optical recording medium 1 c is a CDoptical recording medium having the substrate thickness of 1.2 mm. Anyone of these optical recording media 1 a, 1 b or 1 c is mounted on arotating mechanism not illustrated and rotated at high speed at the timeof recording, playback or erasing.

Here, it should be noted that the aperture 21 can be formed on a bobbinthat holds the objective lens 2 on the actuator that moves the objectivelens 2 in the focusing direction or tracking direction, and it is notnecessary to provide a separate optical component for this purpose.

Hereinafter, operation of the optical system will be explained for eachof the wavelength bands.

First, explanation will be made for the case of carrying out recording,playback or erasing of information to and from the optical recordingmedium 1 a having the substrate thickness of 0.6 mm at the side wherethe optical irradiation is made while using optical source 12 thatproduces the blue wavelength λ1 of 405 nm together with the numericalaperture value NA of 0.65.

The divergent optical beam of the 405 nm wavelength emitted from thelaser diode 12 with linear polarization is converted to a generallyparallel beam by the collimator lens 15 and is deflected by thedeflection prism 19 by the angle of 90 degrees after passing through thepolarization bam splitter 16 and the dichroic prisms 17 and 18.

The optical beam is then converted to a circular polarization beam uponpassage through the quarter wavelength plate 20 and is passed throughthe aperture 21. Thereby, the numerical aperture is restricted to thenumerical aperture value NA of 0.65 by the aperture switching means 22and is incident to the objective lens 2A. Thereby, the objective lens 2Afocuses the optical beam on the optical recording medium 1 a in the formof a minute spot. With this spot, playback, recording or erasing ofinformation is performed.

The optical beam thus focused on the optical recording medium 1 a isreflected by the optical recording medium 1 a, and there is formed areflection optical beam of generally parallel beam having a circularpolarization of opposite rotating direction, and the reflection opticalbeam thus formed is converted to a linear polarization beam having apolarization plane perpendicularly to the polarization plane incident tothe optical recording medium upon passage through the quarter wavelengthplate 20 in the reverse direction.

The linear polarized reflection optical beam thus formed is then passedthrough the deflection prism 19 and the dichroic prisms 18 and 17consecutively and is reflected to the detection lens 23 by thepolarization beam splitter 16. Thereby, the reflected optical beam isdivided into plural optical paths by the beam splitter 24, and detectionof information and servo signal is achieved by the photodetector 25.

Next, recording, playback or erasing of information to and from the DVDoptical recording medium 1 b having the thickness of 0.6 mm at the sidewhere the optical irradiation is made by using the red optical beam ofthe 660 nm wavelength from the optical source 13 with the numericalaperture value NA of 0.65 will be explained.

The optical beam of the 660 nm wavelength emitted from the laser diodechip 13 in the hologram unit is passed through the hologram 33 and isconverted to a predetermined divergent beam by the coupling lens 28.

Thereafter, the optical beam is reflected toward the polarization prism19 by the dichroic prism 17 that passes through the blue wavelength beambut reflects the red optical beam selectively, wherein the optical beamis deflected by the deflection prism 19 by 90 degrees after passingthrough the dichroic prism 14. Thereafter, the optical beam is convertedto a circular polarization beam upon passage through the quarterwavelength plate 20, and the numerical aperture value NA is restrictedto 0.65 at the aperture 21. The optical beam is then incident to theobjective lens 2A and is focused upon the optical recording medium 1 bin the form of minute spot. Playback, recording or erasing ofinformation is achieved by using this optical spot.

Further, the optical beam reflected by the optical recording medium 1 bis deflected by the deflection prism and is reflected by the dichroicprism 17 after passing through the dichroic prism 18. The optical beamis then converted to a converging beam by the coupling lens 28 and isdiffracted by the hologram 22 toward the photodetection device 34provided in the same can in which the laser diode 13 is provided.Thereby, the photodetection device 34 detects the information signal andthe servo signal.

Next, recording, playback or erasing of information to and from the CDoptical recording medium 1 c having the thickness of 1.2 mm at the sidewhere the optical irradiation is made by using the red optical beam ofthe 785 nm wavelength from the optical source 14 with the numericalaperture value NA of 0.50 will be explained.

The optical beam of the 785 nm wavelength emitted from the laser diodechip 14 in the hologram unit is passed through the hologram 35 and isconverted to a predetermined divergent beam by the coupling lens 31.

Thereafter, the optical beam is reflected toward the polarization prism19 by the dichroic prism 18 that passes through the blue wavelength beambut reflects the infrared optical beam selectively, wherein the opticalbeam is deflected by the deflection prism 19 by 90 degrees after passingthrough the dichroic prism 18. Thereafter, the optical beam is convertedto a circular polarization beam upon passage through the quarterwavelength plate 20, and the numerical aperture value NA is restrictedto 0.65 at the aperture 21. The optical beam is then incident to theobjective lens 2A and is focused upon the optical recording medium 1 cin the form of minute spot. Playback, recording or erasing ofinformation is achieved by using this optical spot.

Further, the optical beam reflected by the optical recording medium 1 bis deflected by the deflection prism and is reflected by the dichroicprism 17 after passing through the dichroic prism 18. The optical beamis then converted to a converging beam by the coupling lens 28 and isdiffracted by the hologram 22 toward the photodetection device 34provided in the same can in which the laser diode 13 is provided.Thereby, the photodetection device 34 detects the information signal andthe servo signal.

Further, the optical beam reflected by the optical recording medium 1 cis deflected by the deflection prism and is reflected by the dichroicprism 18. The optical beam is then converted to a converging beam by thecoupling lens 31 and is diffracted by the hologram 35 toward thephotodetection device 36 provided in the same can in which the laserdiode 14 is provided. Thereby, the photodetection device 36 detects theinformation signal and the servo signal.

Hereinafter, the aperture switching part in the vicinity of theobjective lens 2A will be explained.

First, the beam diameter of the incident optical beam for the case ofDVD will be explained.

When a red optical beam is incident to an objective lens optimized toprovide an optimum wavefront in the blue wavelength band with the samebeam diameter, the refraction power of the lens is decreased and thereoccurs decrease of the numerical aperture value NA.

In view of this, the present embodiment uses a beam diameter φ2 in thered wavelength band larger than the beam diameter φ1 used in the bluewavelength band.

FIG. 40 shows the relationship between the wavelength and the effectivediameter for the case of obtaining the numerical aperture value NA of0.65 in the objective lens having the following characteristics in theblue wavelength band

-   φ: 3.965 mm-   NA: 0.65-   F: 3.05 mm-   glass type: 500000.600000,-   for the case a different wavelength is used.

From FIG. 40, it can be seen that it is necessary to set the beamdiameter φ2 to about 4.095 mm in the case of carrying out recording,playback or erasing of information for the DVD recording medium 1 b byusing the wavelength of 660 nm. Further, the wavefront aberration isoptimized in the case of using the wavelength other than 405 nm bychanging the object distance.

Next, explanation will be made on the incident optical beam diameterused for the case of CD.

In the case of carrying out recording, playback or erasing to or fromthe CD optical recording medium 1 c, the optimum aperture number NA isabout 0.5. On the other hand, the optimum effective beam diameter φ3 forsuch a case of CD is estimated to be about 3.28 mm by the similarprocess explained with reference to FIG. 40.

Thus, it is necessary to carry out three-step switching for the aperturefor achieving the beam diameter of any of φ1, φ2 or φ3 for the opticalbeam incident to the objective lens 2A. In the present embodiment, theincident optical beam diameter φ2 is restricted by the aperture 21provided to the actuator part, and a wavelength selective diffractionelement formed with a wavelength-selective diffraction pattern is usedas the aperture switching element 22 that switches the optical beamdiameters φ1 and φ3 in response to the wavelength of the optical beamemitted from the optical source as shown in FIGS. 41A-41C.

The aperture switching means 22 has a central region having a diameterφ3 where there is provided no wavelength-selective diffraction elementwhile this central region is surrounded by a wavelength-selectivediffraction element 41. Reference should be made to FIGS. 41A-41C.

Thus, the optical beam of any of the blue wavelength and the redwavelength having the beam diameter φ1 or φ2 can pass through theaperture switching element 22 freely as shown in FIGS. 41A and 41B,while the infrared optical beam is reflected back by thewavelength-selective diffraction element 41 surrounding the centralregion, and the beam size of the infrared optical beam is restricted tothe beam diameter of φ3.

Of course, the aperture switching means 22 is not limited to the onehaving the wavelength-selective diffraction element 41 but may be adevice having a wavelength-selective transmission/reflection coating.

Eight Embodiment

Next, the application of an optical pickup 11 to the NA0.70blue/DVD/CDtechnologies will be described as an eighth embodiment of the presentinvention.

It should be noted that the optical pickup of the present embodiment isan optical pickup having an optical source 12 of the wavelength λ1 of405 nm for the blue wavelength band, an optical source 13 of thewavelength λ2 of 660 nm for the red wavelength band, and an opticalsource 14 of the wavelength λ3 of 785 nm for the infrared wavelengthband, wherein the optical pickup is capable of carrying out recording,playback and erasing to and from any of: the blue optical recordingmedium 1 a having the substrate thickness of 0.6 mm at the sideirradiated with the optical beam and designed for use with the numericalaperture value NA of 0.70; the DVD medium having the substrate thicknessof 0.6 mm at the side irradiated with the optical beam and designed foruse with the numerical aperture value NA of 0.65; and the CD mediumhaving the substrate thickness of 1.2 mm at the side irradiated with theoptical beam and designed for use with the numerical aperture value NAof 0.50.

The difference of the present embodiment over the seventh embodiment isthat the numerical aperture value of 0.70 is used for the blue opticalsystem in place of the numerical aperture value of 0.65, and thus, thebeam diameter of the blue optical beam is increased as compared with thecase of the red optical beam used for DVD.

Thus, although the overall construction is identical to that of FIG. 39,the present embodiment uses the objective lens 1B in place of theobjective lens 1A. It should be noted that, with the numerical aperturevalue NA of about 0.70, the degradation of tolerance caused by variousvariations is not significant and it is possible to increase the storagecapacity of the optical recording medium. By using the numericalaperture value of 0.70, the storage capacity can be increased by about15% as compared with the case of using the numerical aperture value of0.65.

In the present embodiment, the incident optical beam diameter φ1 isrestricted by the aperture provided on the actuator part, while theaperture switching means 22 carries a wavelength-selective diffractionelement 42 having a wavelength-selective diffraction pattern forchanging the beam diameter of the optical beam emitted from the opticalsource between the beam diameter φ2 and the beam diameter φ3 as shown inFIGS. 42A-42C.

Referring to FIG. 42A, the blue optical beam having the beam diameter φ1passes freely through the wavelength-switching means 22 in view of thefact that the wavelength-selective diffraction element 42 does not causediffraction in the blue optical beam.

In the case of the red optical beam shown in FIG. 42B, on the otherhand, the incident optical beam is reflected by the wavelength-selectivediffraction element 42 in the part thereof outside the region having thediameter φ2, and the beam diameter of the red optical beam is restrictedto the diameter of φ2.

In the case of the infrared optical beam showing in FIG. 42C, theincident optical beam is reflected by the wavelength-selectivediffraction element 42 in the part hereof outside the region having thediameter φ3, and the beam diameter of the infrared optical beam isrestricted to the diameter of φ3.

Ninth Embodiment

Next, the application of an optical pickup 11 to the NA0.67blue/DVD/CDtechnologies will be described as an eighth embodiment of the presentinvention.

It should be noted that the optical pickup of the present embodiment isan optical pickup having an optical source 12 of the wavelength λ1 of405 nm for the blue wavelength band, an optical source 13 of thewavelength λ2 of 660 nm for the red wavelength band, and an opticalsource 14 of the wavelength λ3 of 785 nm for the infrared wavelengthband, wherein the optical pickup is capable of carrying out recording,playback and erasing to and from any of: the blue optical recordingmedium 1 a having the substrate thickness of 0.6 mm at the sideirradiated with the optical beam and designed for use with the numericalaperture value NA of 0.67; the DVD medium having the substrate thicknessof 0.6 mm at the side irradiated with the optical beam and designed foruse with the numerical aperture value NA of 0.65; and the CD mediumhaving the substrate thickness of 1.2 mm at the side irradiated with theoptical beam and designed for use with the numerical aperture value NAof 0.50.

The difference of the present embodiment over the seventh embodiment isthat the numerical aperture value of 0.67 is used for the blue opticalsystem in place of the numerical aperture value of 0.65, and thus, thebeam diameter of the blue optical beam is increased as compared with thecase of the red optical beam used for DVD.

Thus, although the overall construction is identical to that of FIG. 39,the present embodiment uses the objective lens 1C in place of theobjective lens 1A.

When a red optical beam is incident to an objective lens optimized toprovide an optimum wavefront in the blue wavelength band with the samebeam diameter, the refraction power of the lens is decreased and thereoccurs decrease of the numerical aperture value NA. This means thatthere occurs increase of the numerical aperture value NA when theoptical beam of the blue wavelength is used as compared with the casethe optical beam of the red wavelength band is used, and a largecapacity optical pickup is realized together with the effect ofreduction of the wavelength. On the other hand, the numerical aperturevalue NA for the DVD generation is determined to be about 0.65.

FIG. 43 shows the relationship between the numerical aperture values NAfor the case a blue optical beam having the 405 nm wavelength and a redoptical beam of the 660 nm wavelength are incident to the objective lenshaving the following characteristics:

focal distance F at the blue wavelength 3.05 mm glass type:550000.550000,

optical recording medium substrate thickness: 0.6 mm.

From FIG. 43, it can be seen that, in the case the numerical aperturevalue NA of 0.65 is achieved for the red optical beam for DVD, thenumerical aperture value NA of about 0.67 can be achieved for theoptical beam of the blue wavelength and having the same beam diameter.

Thus, by using such a relationship, it is possible to use the two-stepaperture switching similar to the one used for the compatibility betweenthe DVD generation and the CD generation, not the three-step apertureswitching as in the case of the fourth or fifth embodiment of thepresent invention. Thereby simplification of the device is achieved.

For the aperture switching means 22 of the present embodiment, it ispossible to use a wavelength-selective diffraction element 43 includinga wavelength-selective diffraction pattern for the switching of theoptical beam diameter as shown in FIGS. 44A-44C in response to thewavelength of the optical beam emitted from the optical source.

Referring to FIGS. 44A and 44B, the blue optical beam having thediameter φ1 or the red optical beam having the diameter φ2 can passfreely through the aperture switching element 22 while the infraredoptical beam is reflected by the wavelength-selective diffractionelement 43 at the region outside the central part having the diameterφ3, and thus, the diameter of the infrared optical is restricted to theforegoing diameter φ3.

FIG. 45 shows the relationship between the incident beam diameter forthe case the red optical beam of the 660 nm wavelength is passed throughthe objective lens 2C of the present embodiment (X-axis) and thedistance between the aperture switching means 22 and the front-sideprincipal point of the objective lens 2C (first Y-axis at the left) forthe case of achieving the numerical aperture value NA of 0.65 withregard to the incident optical beam radius. Further, the second Y axisat the right of FIG. 45 shows the numerical aperture value NA for thecase the blue optical beam of the 405 nm wavelength is passed throughthe same lens as the function of the incident optical beam diameter.

In view of the use of the numerical aperture value NA of 0.67 for thecase of using the blue optical recording medium 1 a in the presentembodiment, the optimum beam diameter φ for the incident optical beamand the optimum distance t between the aperture switching means 22 andthe front side principal point of the objective lens 2C is given asφ=4.1 mm and t=0 mm.

Hereinafter, the relationship of FIG. 45 will be explained.

In the present embodiment, the optical pickup includes the first opticalsource 12 producing the first optical beam with the first wavelength λ1,the second optical source 13 producing the second optical beam with thesecond wavelength λ2, and the single objective lens 2C that focuses anyof the first and second optical beams to the optical recording medium 1,wherein the first optical beam of the 405 nm wavelength is used with aninfinite optical system upon activation of the first optical source 12and the second optical beam of the 660 nm wavelength is used with thefinite optical system upon activation of the second optical source 13,wherein the predetermined aperture is provided with an offset from theprincipal point at the front side (incident side) of the objective lenswith the distance t in the direction toward the optical source given ast≈L−NA1·f/tan(a sin(NA2obj))where f represents the focal distance of the objective lens, NA1represents the numerical aperture value at the side of the image surfaceat the time the optical source 12 of the wavelength λ1 (=405 nm) isturned on, NA2obj represents the numerical aperture value at the side ofthe optical source at the time the optical source 13 of the wavelengthλ2 (=660 nm) is turned on, and the L represents the object distance atthe time the optical source 13 of the wavelength λ2 (=660 nm) is turnedon. As a result, switching of the aperture between the case of using thewavelength of 405 nm and the case of using the wavelength of 660 nm isnot necessary.

Here, the foregoing relationshipt≈L−NA1·f/tan(a sin(NA2obj))will be explained with reference to FIG. 46.

Referring to FIG. 46, the optical path of the first optical beam of thefirst wavelength λ1 incident to the objective lens 1C via the infiniteoptical system and focused upon the optical recording medium with thenumerical aperture value NA1 is represented by the one-dotted chainline.

Generally, it is known that there exists a relationshipNA1=φ/2/fbetween the focal distance f of the objective lens 1C, the numericalaperture value NA1 and the incident beam diameter φ.

On the other hand, the optical path of the second optical beam of thesecond wavelength λ2 incident to the objective lens 1C via the finiteoptical system and focused upon the optical recording medium with thenumerical aperture value NA2 is represented by the one-dotted chainline.

Thus, in the case the aperture is disposed at the location offset fromthe front side principal point of the objective lens such that thesecond optical beam of the wavelength λ2 is φ, the relationshipφ/2=(L−t)×tan(a sin(NA2obj))is obtained. By substituting this to the equation for NA1, the foregoingequation for t is obtained.

Incidentally, the present embodiment uses the construction of λ1=405 nm,NA1=0.67 for the blue optical beam and the construction of λ2=660 nm,NA2=0.65, L=142 mm, NA2obj=0.015 for the red optical beam and φ=4.1 mm,t=0.0 mm.

Next, the example of using the object distance determined so as tominimize the wavefront aberration in the case of using the finiteoptical system for the DVD technology and the CD technology will beexplained.

For example, in the case of the sixth embodiment for the objective lens,the relationship between the object distance and the wavefrontaberration is represented as shown in FIG. 47 when the optical beam ofthe 660 nm for the DVD is used. Thus, in this case the object distanceof 142 mm is used so as to minimize the wavefront aberration.

For the aperture switching means 22, not only the one that usesdiffraction but also the elements that use reflection or absorption orpolarization may be used.

In the case of using reflection, a dielectric multilayer film havingwavelength selectivity may be provided similarly to the case of thewavelength-selective diffraction element 43.

More specifically, it is possible to construct the aperture switchingmeans 22 such that the central part where the dielectric multilayer filmis not provided shows high transmissivity to any of the blue, red andinfrared wavelength bands while the region outside the central regionshows a high transmissivity only to the blue and red wavelength bandsand shows low transmissivity to the infrared optical wavelength band.

Similarly, it is possible to change the beam diameter in the apertureswitching means 22 in response to the wavelength thereof by usingoptical absorption.

Further, it is possible to use polarization in the aperture switchingelement 22 for changing the beam diameter in response to the wavelength.In this case, the red optical beam and the infrared optical beam aredisposed such that the polarization directions intersect perpendicularlywith each other. Thereby, the aperture is switched by switching thepolarization direction between the mutually perpendicular polarizationdirections.

For the aperture switching means 22, it is also possible to provide acoating or diffraction grating on the surface of the objective lens 2(2A, 2B, 2C), not in the form of a separate element as shown in FIGS.41A-41C.

For example, FIGS. 48A-48C show the case of forming thetransmission/reflection coating 44 having wavelength selectivity on thelens surface of the objective lens 2 at the side facing the opticalsource.

Thus, FIG. 48A shows the case the optical beam having the wavelength of405 nm is irradiated upon the blue optical recording medium 1 a, whileFIG. 48B shows the case the optical beam having the wavelength of 660 nmis irradiated upon the DVD medium 1 b and FIG. 18C shows the case theoptical beam having the wavelength of 785 nm is irradiated upon the CDmedium 1 c.

Here, the coating 44 passes the optical radiation at the wavelength of405 nm and 660 nm while it reflects the optical radiation at thewavelength of 785 nm selectively.

Tenth Embodiment

Next, the construction of an optical information processing apparatusaccording to a tenth embodiment of the present invention will beexplained with reference to FIG. 49.

Referring to FIG. 49, the information processing apparatus 51 is anapparatus carrying out at least one of recording, playback and erasingof information to and from an optical recording medium by using anoptical pickup 52 shown in FIG. 39.

In the present embodiment, it should be noted that the optical recordingmedium 1 is implemented in the form of a disk accommodated in acartridge 53 or protective case. Thus, the optical recording medium 1 isloaded to the information recording apparatus 51 by inserting the disk 1into an insertion opening 54 of the information recording apparatus 51in the direction of an arrow together with the protective case 53. Thedisk 1 thus loaded is rotated by a spindle motor 55, and recording,playback or erasing of information is achieved by the optical pickup 52.Further, it should be noted that the optical recording medium 1 (1 a, 1b or 1 c) is not necessarily accommodated in the protective case but maybe used in the bare state.

For the optical pickup 52, it is possible to use the optical pickupdescribed in the foregoing embodiments, and the optical informationprocessing apparatus 51 can achieve the three-generation compatibilitybetween blue/DVD/CD technologies without using an aperture switchingelement (or two-generation compatibility between blue/DVD technology) byusing the objective lens or optical pickup in which the sphericalaberration is effectively suppressed.

Further, the present invention is by no means limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

1. An optical pickup comprising: a first optical source producing afirst optical beam having a first wavelength λ1; a second optical sourceproducing a second optical beam having a second wavelength λ2; a thirdoptical source producing a third optical beam having a third wavelengthλ3; and an objective lens focusing any of said first, second and thirdoptical beams on an optical recording medium, said objective lens havinga single-lens construction and defined by a non-spherical convex surfaceat an incident side and an exit side, wherein the objective lenssatisfies a condition:νd>35  (1)1.58>nd  (2)0.58nd−0.29≦R1/f≦0.62nd−0.31  (3) where nd and νd represent respectivelythe refractive index and Abbe number for a D-line, R1 represent theparaxial radius of curvature and f represents the focal distance of theobjective lens, said objective lens having a first effective numericalaperture value NA(λ1) for said first optical beam, a second effectivenumerical aperture value NA(λ2) for said second optical beam, and athird effective numerical aperture value NA(λ3) for said third opticalbeam, said first optical beam incident to said objective lens having afirst beam diameter φ1, said second optical beam incident to saidobjective lens having a second beam diameter φ2, said third optical beamincident to said objective lens having a third beam diameter φ3, saidfirst, second and third effective numerical aperture values NA(λ1),NA(λ2), NA(λ3) and said first, second and third beam diameters φ2, φ1,φ3 satisfying a relationshipNA(λ1)=NA(λ2)>NA(λ3)  (4)φ2>φ1>φ3.  (5)